Showing papers in "Journal of Molecular Evolution in 1983"

Accuracy of estimated phylogenetic trees from molecular data. II. Gene frequency data.

Abstract: The accuracies and efficiencies of three different methods of making phylogenetic trees from gene frequency data were examined by using computer simulation. The methods examined are UPGMA, Farris' (1972) method, and Tateno et al.'s (1982) modified Farris method. In the computer simulation eight species (or populations) were assumed to evolve according to a given model tree, and the evolutionary changes of allele frequencies were followed by using the infinite-allele model. At the end of the simulated evolution five genetic distance measures (Nei's standard and minimum distances, Rogers' distance, Cavalli-Sforza's fλ, and the modified Cavalli-Sforza distance) were computed for all pairs of species, and the distance matrix obtained for each distance measure was used for reconstructing a phylogenetic tree. The phylogenetic tree obtained was then compared with the model tree. The results obtained indicate that in all tree-making methods examined the accuracies of both the topology and branch lengths of a reconstructed tree (rooted tree) are very low when the number of loci used is less than 20 but gradually increase with increasing number of loci. When the expected number of gene substitutions (M) for the shortest branch is 0.1 or more per locus and 30 or more loci are used, the topological error as measured by the distortion index (dT) is not great, but the probability of obtaining the correct topology (P) is less than 0.5 even with 60 loci. When M is as small as 0.004, P is substantially lower. In obtaining a good topology (small dT and high P) UPGMA and the modified Farris method generally show a better performance than the Farris method. The poor performance of the Farris method is observed even when Rogers' distance which obeys the triangle inequality is used. The main reason for this seems to be that the Farris method often gives overestimates of branch lengths. For estimating the expected branch lengths of the true tree UPGMA shows the best performance. For this purpose Nei's standard distance gives a better result than the others because of its linear relationship with the number of gene substitutions. Rogers' or Cavalli-Sforza's distance gives a phylogenetic tree in which the parts near the root are condensed and the other parts are elongated. It is recommended that more than 30 loci, including both polymorphic and monomorphic loci, be used for making phylogentic trees. The conclusions from this study seem to apply also to data on nucleotide differences obtained by the restriction enzyme techniques.

Radiation of human mitochondria DNA types analyzed by restriction endonuclease cleavage patterns

Abstract: Human mitochondrial DNA (mtDNA) restriction endonuclease fragment patterns were analyzed using total blood cell DNA isolated from 200 individuals representing five different populations. Thirty-two fragment patterns (morphs) were observed with the enzymes Hpa I, Bam HI, Hae II, Msp I and Ava II yielding thirty-five different combinations of fragment patterns (mt DNA types). The major ethnic groups exhibit quantitative as well as qualitative differences in their mtDNA types, all of which are related to each other by a tree in which the closely related mtDNA types cluster according to geographic origin.

Biological diversity, chemical mechanisms, and the evolutionary origins of bioluminescent systems

Abstract: A diversity of organisms are endowed with the ability to emit light, and to display and control it in a variety of ways. Most of the luciferins (substrates) of the various phylogenetically distant systems fall into unrelated chemical classes, and, based on still limited data, the luciferases (enzymes) and reaction mechanisms are distinctly different. Based on its diversity and phylogenetic distribution, it is estimated that bioluminescence may have arisen independently as many as 30 times in the course of evolution. However, there are several examples of cross-phyletic similarities among the substrates; some of these may be accounted for nutritionally, but in other cases they may have evolved independently.

Prebiotic synthesis in atmospheres containing CH4, CO, and CO2. I. Amino acids.

Abstract: The prebiotic synthesis of organic compounds using a spark discharge on various simulated primitive earth atmospheres at 25 degrees C has been studied. Methane mixtures contained H2 + CH4 + H2O + N2 + NH3 with H2/CH4 molar ratios from 0 to 4 and pNH3 = 0.1 torr. A similar set of experiments without added NH3 was performed. The yields of amino acids (1.2 to 4.7% based on the carbon) are approximately independent of the H2/CH4 ratio and whether NH3 was present, and a wide variety of amino acids are obtained. Mixtures of H2 + CO + H2O + N2 and H2 + CO2 + H2O + N2, with and without added NH3, all gave about 2% yields of amino acids at H2/CO and H2/CO2 ratios of 2 to 4. For a H2/CO2 ratio of 0, the yield of amino acids is extremely low (10(-3)%). Glycine is almost the only amino acid produced from CO and CO2 model atmospheres. These results show that the maximum yield is about the same for the three carbon sources at high H2/carbon ratios, but that CH4 is superior at low H2/carbon ratios. In addition, CH4 gives a much greater variety of amino acids than either CO or CO2. If it is assumed that an abundance of amino acids more complex than glycine was required for the origin of life, then these results indicate the requirement for CH4 in the primitive atmosphere.

Prebiotic synthesis in atmospheres containing CH4, CO, and CO2. II. Hydrogen cyanide, formaldehyde and ammonia.

Abstract: The electric discharge synthesis of HCN, H2CO, NH3 and urea has been investigated using various mixtures of CH4, CO, CO2, N2, NH3, H2O, and H2. HCN and H2CO were each synthesized in yields as high as 10% from CH4 as a carbon source. Similar yields were obtained from CO when H2/CO greater than 1.0 and from CO2 when H2/CO2 greater than 2.0. At H2/CO2 less than 1.0 the yields fall off drastically. Good yields of NH3 (0.7 to 5%) and fair yields of urea (0.02 to 0.63%) based on nitrogen were also obtained. The directly synthesized NH3 together with the NH3 obtained from the hydrolysis of HCN, nitriles and urea could have been a major source of ammonia in the atmosphere and oceans of the primitive earth. These results show that prebiotic syntheses from HCN and H2CO to give products such as purines and sugars and some amino acids could have occurred in primitive atmospheres containing CO and CO2 provided the H2/CO and H2/CO2 ratios were greater than about 1.0. Methane containing atmospheres give comparable quantities of HCN and H2CO, and are superior in the synthesis of amino acids.

What is a conservative substitution

Abstract: It is commonly recognised that many evolutionary changes of amino acid sequence in proteins are conservative: a substitution of one amino acid residue for another has a far greater chance of being accepted if the two residues are similar in properties. Here we investigate what properties are most important in determining the similarity of two amino acids, from the evolutionary point of view. Our results confirm earlier observations that the hydrophobicity and the molecular bulk of the side chain tend to be conserved. More importantly they also show that evolutionary pressures favour the conservation of secondary structure, and that all these properties can be arranged in a two dimensional diagram in which distances well preserve the observed substitution frequencies between amino acids. These results were obtained by a multi-dimensional scaling technique; and are independent of any prior opinions about conserved properties. Thus, it is demonstrated that all relations of importance to single amino acid substitutions can be represented by a single figure, which is much more comprehensible and useful than the usual tabular representation of substitution frequencies. Such a figure conveniently portrays the “stereochemical code” for conservative substitution.

Transitions and transversions in evolutionary descent: An approach to understanding

Abstract: A quantitative theoretical groundwork is presented for determining the proportions of the possible types of base substitutions observed between 12 genes sharing a common ancestor and isolated from extant species. Three methods (direct count, regression, and informational entropy maximization) are described by which conditional base substitution probabilities that determine evolutionary descent can be estimated from experimental data. These methods are utilized to study the ratio of transversions to transitions during gene divergence. The limiting ratio is directly calculated from a knowledge of the 12 conditional probabilities for each type of base substitution and from a knowledge of the equilibrium base composition of the DNAs compared. An expression is developed for this calculation. It is concluded that multiple substitutions per se do not lead to a decrease in transition differences with increasing evolutionary divergence.

Many gene-regulatory proteins appear to have a similar α-helical fold that binds DNA and evolved from a common precursor

Abstract: Amino acid and DNA sequence comparisons suggest that many sequence-specific DNA-binding proteins have in common and homologous region of about 22 amino acids. This region corresponds to two consecutive α-helices that occur in bot Cro and cI repressor proteins of bacteriophage λ and in catabolite gene activator protein ofEscherichia coli and are presumed to interact with DNA. The results obtained here suggest that this α-helical DNA-binding fold occurs in many proteins that regulate gene expression. It also appears that this DNA-binding unit evolved from a common evolutionary precursor.

The protein burden of lac operon products.

Abstract: A new approach to measuring the slowing of growth due to the manufacture of proteins not needed by a bacterium is presented. An entire single colony of Escherichia coli was used to start a chemostat culture that was then given a selective pressure by the addition of phenylgalactoside (phi-gal). This enriched the population for constitutive mutants that produced beta-galactosidase without induction and could split phi-gal, consume the galactose, and grow faster. When the phi-gal was removed, the constitutives grew slower than the parental strain and were gradually lost. This procedure allows competition experiments to be carried out with minimum effects due to genetic drift. Experiments with both strains having wild-type and mutant permease genes were conducted. With the former the selective disadvantage was initially much greater than expected from the simplest hypothesis that extra unused proteins would slow growth in proportion to their fraction of the total protein synthesis. This phase was followed by a second phase where the selective disadvantage was smaller than predicted by this simple hypothesis. With a very slowly reverting permease negative strain the selective disadvantage, and therefore the protein burden, was found to be much smaller and not statistically different from zero. Thus, while one would expect under carbon and energy limitation in the chemostat the protein burden to be larger than under unlimited conditions, it is so small that even the refined technique used here could not measure it accurately. It is certainly less than the fraction of 'waste' protein synthesis; but it could be between zero and the fraction of the cells' energy and carbon budget spent on manufacture of the proteins of the lac operon.

Evolution of Drosophila repetitive-dispersed DNA.

Abstract: We have examined the phylogenetic distribution of a spectrum ofDrosophila repetitive-dispersed DNAs ranging from structurally complex transposable elements to scrambled middle repetitive sequences. Our data suggest that unlike typical “genes” these DNAs are unstable components of the drosophilid genome. The unusual behavior of these repetitive-dispersed DNAs raises the possibility that this type of sequence may have an important role in the evolution of the family Drosophilidae.

The evolutionary pattern of aromatic amino acid biosynthesis and the emerging phylogeny of pseudomonad bacteria

Abstract: Pseudomonad bacterial are a phylogenetically diverse assemblage of species named within contemporary genera that includePseudomonas, Xanthomonas andAlcaligenes. Thus far, five distinct rRNA homology groups (Groups I through V) have been established by oligonucleotide cataloging and by rRNA/DNA hybridization. A pattern of enzymic features of aromatic amino acid biosynthesis (enzymological patterning) is conserved at the level of rRNA homology, five distinct and unambiguous patterns therefore existing in correspondence with the rRNA homology groups. We sorted 87 pseudomonad strains into Groups (and Subgroups) by aromatic pathway patterning. The reliability of this methodology was tested in a blind study using coded cultures of diverse pseudomonad organisms provided by American Type Culture Collection. Fourteen of 14 correct assignments were made at the Group level (the level of rRNA homology), and 12 of 14 correct assignments were made at the finer-tuned Subgroup levels. Many strains of unknown rRNA-homology affiliation had been placed into tentative rRNA groupings based upon enzymological patterning. Positive confirmation of such strains as members of the predicted rRNA homology groups was demonstrated by DNA/rRNA hybridization in nearly every case. It seems clear that the combination of these molecular approaches will make it feasible to deduce the evolution of biochemical-pathway construction and regulation in parallel with the emerging phylogenies of microbes housing these pathways.

Phylogeny of protozoa deduced from 5S rRNA sequences

Abstract: The nucleotide sequences of 5S rRNAs from three protozoa,Bresslaua vorax, Euplotes woodruffi andChlamydomonas sp. have been determined and aligned together with the sequences of 12 protozoa species including unicellular green algae already reported by the authors and others. Using this alignment, a phylogenic tree of the 15 species of protozoa has been constructed. The tree suggests that the ancestor for protozoa evolved at an early time of eukaryotic evolution giving two major groups of organisms. One group, which shares a common ancestor with vascular plants, contains a unicellular green flagellate (Chlamydomonas) and unicellular green algae. The other group, which shares a common ancestor with the multicellular animals, includes various flagellated protozoa (includingEuglena), ciliated protozoa and slime molds. Most of these protozoa appear to have separated from one another at a fairly early period of eukaryotic evolution.

The spatial distribution of fixed mutations within genes coding for proteins

Abstract: An examination has been conducted of the extensive amino acid sequence data now available for five protein families - the alpha crystallin A chain, myoglobin, alpha and beta hemoglobin, and the cytochromes c - with the goal of estimating the true spatial distribution of base substitutions within genes that code for proteins. In every case the commonly used Poisson density failed to even approximate the experimental pattern of base substitution. For the 87 species of beta hemoglobin examined, for example, the probability that the observed results were from a Poisson process was the minuscule 10 to the -44th. Analogous results were obtained for the other functional families. All the data were reasonably, but not perfectly, described by the negative binomial density. In particular, most of the data were described by one of the very simple limiting forms of this density, the geometric density. The implications of this for evolutionary inference are discussed. It is evident that most estimates of total base substitutions between genes are badly in need of revision.

A prevalent persistent global nonrandomness that distinguishes coding and non-coding eucaryotic nuclear DNA sequences

Abstract: Coding sequences of eucaryotic nuclear DNA were characterized by an excess of short runs and a deficit of long runs of weak and of strong hydrogen bonding bases; non-coding sequences by a deficit of short runs and an excess of long runs, in the same of purines and of pyrimidines. The conservation of these attributes across DNA sequences coding for proteins of widely different function, across widely different eucaryotic species for the same protein and across related genes that diverged a long time ago and that now show large differences in base and, if coding, amino acid sequence suggested that these attributes have survival value. It was concluded that these attributes constitute probalistic constraints on th primary structure (base sequence) of both coding and non-coding DNA.

An empirical evaluation of qualitative Hennigian analyses of protein electrophoretic data.

Abstract: In an empirical evaluation of a qualitative approach to construction of phylogenetic trees from protein-electrophoretic data, we have employed Hennigian cladistic principles to generate molecular trees for waterfowl, rodents, bats, and other phylads. This procedure of tree construction is described in detail. Branching structures of molecular trees produced by three different algorithms were compared against those of “model” classifications previously proposed by other systematists. In each case, the qualitative cladistic trees provided fits to model phylogenies which were strong and as good or better than those resulting from phenetic-clustering or distance-Wagner trees based on manipulation of quantitative values in matrices of genetic distance.

Evolution of alpha q- and beta-tubulin genes as inferred by the nucleotide sequences of sea urchin cDNA clones.

Abstract: Evolutionary studies on the tubulin multigene families were initiated by nucleotide sequence analysis of cDNA clones complementary to sea urchin (Lytechinus pictus) tesits α- and β-tubulin mRNAS. Sequence comparisons of three partial β-tubulin cDNA clones (pβ1, pβ2, pβ3) demonstrated the existence of tubulin mRNA heterogeneity. pβ2 and pβ3 contain identical tubulin-coding regions and extremely similar 3′ untranslated sequences, including a polyadenylation signal (AAUAAA). However, pβ2 contains an additional region of 3′ untranslated sequence which includes a second plyadenylation signal. These two sequences may be allelic, representing products of alternative transcription termination or processing pathways. pβ1 and pβ2 (or pβ3) cDNAs almost certainly correspond to transcripts of distinct but evolutionarily related genes. Examination of the available coding portions showed that they differ only by a few silent nucleotide substitutions and the deletion/insertion of one codon; most of the differences are clustered within the last 15 3′-end codons. In contrst, their 3′ untranslated sequences are considerably divergent. Nucleotide alignment in this region was feasible by considering specific point and segmental mutations, mainly T↔C transitions and small deletions/insetions associated with small direct repeats. The sea urchin α- and β-tubulin cDNA and corresponding protein sequences were compared with previously described tubulin cDNA and protein sequences from other organisms. Both α and β tubulins are very conserved proteins, evolving with a rete comparable to that of histones. Analysis of the nucleotide divergence of the coding cDNA regions showed that relacement sites have changed with a rate 20–175 times lower than that of the silent sites. Among the 177 codons compared between the sea urchinb testic and chick brain β-tubulin cDNAs, there are 7 conservative amino acid replacements and the deletion/insertion of two codons. Most of these changes are clustered near the C-terminus. The 161-amino acid portion of chick brain, rat and porcine α-tubulin sequences differs by 3 conservative amino acid replacements from the corresponding sea urchin testis α-tubulin sequence. The compared interspecies 3′ untranslated sequences are very divergent.

Evolution of the amino acid code: inferences from mitochondrial codes.

Abstract: The amino acid code is usually presented as a table of 64 codons. Actually the code results from the action of tRNA molecules that carry amino acids to codons in mRNA by means of codon-anticodon pairing. The tRNA molecules are transcribed from genes that undergo evolution and the number of anticodons can therefore increase during evolution, but the number of codons is fixed at 64. Mammalian mitochondrial codes contain only 22 anticodons for 20 amino acids as compared with 54 anticodons for 20 amino acids in the universal code. It is proposed that an archetypal code containing 16 anticodons for 15 amino acids evolved into the universal code by gene duplication, followed by mutations that modified the anticodons and amino acid acceptor sites. In substantiation of this proposal, it is noted that the mammalian mitochondrial code, is simplified by comparison with the universal code. For example, single anticodons are used for each of eight amino acids in the mammalian mitochondrial code. This simplification may represent an evolutionary retrogression towards the proposed archetypal code.

The frequency and distribution of methylatable DNA sequences in leguminous plant protein coding genes

Abstract: Methylation of higher plant DNA occurs at up to 25% of all cytosines, primarily in the sequences CpG2 and CpNpG, both of which are over 80% methylated in wheat and tobacco (Gruenbaum, et al 1981) CpG and CpNpG frequencies and distributions in the known sequences of cloned genes of leguminous plants were analyzed In this sample CpG occured at only 49% of the frequency expected if the bases were distributed at random This lower frequency may be attributed to the fixation of mutations generated by a high rate of deamination of 5methylcytosine to thymine (Salser 1977) Consistent with this hypothesis, the product of CpG transitions, TpG and CpA, were significantly above their expected frequency However CpNpG occured at approximately expeced levels and there was no significant increase in its transistion products CpNpA and TpNpG Possible explanations for this phenomenon are discussed An analysis of the distribution of di- and trinucleotides across functionally classified regions of genes showed CpG to be asymmetrically distributed CpG was on average significantly enriched in the 3′ flanking regions compared to other regions This may reflect a methylation-mediated regulatory role for this region in some legume genes

Codon usage in muscle genes and liver genes

Abstract: Synonymous codon usage frequencies, derived from cDNA clone sequences, were compared for several sets of vertebrate genes. Gene sets as diverse as those expressed in avian skeletal muscle and in mammalian liver showed similar patterns of synonymous codon usage. There were no significant differences suggesting tissue-specific co-adaptation of codon usage patterns and tRNA anticodon profiles. The results indicate a consensus codon usage pattern for vertebrate genes which is largely independent of taxonomic class, tissue of expression, and the cellular fate and rate of evolution of the encoded proteins. Certain elements of the consensus codon usage pattern indicate that it is the product of natural selection and not simply a mutational equilibrium among phenotypically equivalent synonyms.

tRNA-rRNA sequence homologies: Evidence for a common evolutionary origin?

Abstract: Many tRNAs ofE. coli and yeast contain stretches whose base sequences are similar to those found in their respective rRNAs. The matches are too frequent and extensive to be attributed to coincidence. They are distributed without discernible pattern along and among the RNAs and between the two species. They occur in loops as well as in stems, among both conserved and non-conserved regions. Their distributions suggest that they reflect common ancestral origins rather than common functions, and that they represent true homologies.

Glutathione reductase in evolution.

Abstract: The disulfide reducing activities of GSSG-and CoASSG-reductases were measured on partially purified extracts from a variety of prokaryotes and eukaryotes.

Choice of base at silent codon site 3 is not selectively neutral in eucaryotic structural genes: it maintains excess short runs of weak and strong hydrogen bonding bases.

Abstract: On the average in the coding sequences of 30 eucaryotic structural genes the weak hydrogen bonding, W, (A or T) or strong hydrogen bonding, S, (C or G) base in codon site 3 was chosen to be unlike its neighbors on both sides up to two sites away This preference produced the nonrandom excess of runs W and S of length one and two and the defict of long runs observed earlier (Blaisdell 1982) The neighbors in the different codon, 3′ to codon site 3, were as important in determining the choice as were the neighbors 5′ in the same codon Every amino acid except methionine and tryptophan, of least frequent occurrence, permits choice of W or S The persistence of this preference could explain the observation that the rate of substitution of codon site 3 in fuctional genes is considerably less than in synonymous pseudo genes

Experimental evolution of a novel pathway for glycerol dissimilation inEscherichia coli

Abstract: Wild-typeEscherichia coli utilizes glycerol aerobically through an inducible pathway mediated by an ATP-dependent kinase and a glycerol 3-phosphate dehydrogenase which is a flavoprotein. A mutant, strain ECL424, employing a novel pathway for glycerol utilization was isolated. The novel pathway is mediated by an NAD-linked dehydrogenase and a dihydroxyacetone specific enzyme II of the phosphoenolpyruvate phosphotransferase system. This study describes the selection from strain ECL424, a derivative which grows more rapidly on glycerol. The derivative, strain ECL428, produces twice the parental levels of both the dehydrogenase and the enzyme II during growth on glycerol. The function of the dehydrogenase in wild-type cells is unknown, although hydroxyacetone (acetol), 3-hydroxy-2-butanone (acetoin), and 1-amino-2-propanone are gratuitous inducers. The induction can be prevented by glucose whose effect can be cancelled by external cyclic AMP. The effects of hydroxyacetone, glucose, and cyclic AMP are attenuated in the two mutants in which the dehydrogenase is produced at high basal levels. The dihydroxyacetone specific enzyme II is inducible by the substrate in both wild-type and mutant strains and serves for growth on the triose.

Radiolysis of aqueous solutions of hydrogen cyanide (pH∼6): Compounds of interest in chemical evolution studies

Abstract: Oxygen-free aqueous solutions of hydrogen cyanide, 01 M and pH∼6, were exposed to gamma rays from a60Co source, the mixture of nonvolatile products was fractionated and the fractions were analysed It has been found that the complex mixture contains oligomers and polymers with molecular weights up to 20,000 daltons, mainly polyamides with urea and peptidic fragments Among the constituents are carbamyl glycinonitrile and carbamyl glycinamide that represent 64% and 31% of the total of unfractionated material respectively Urea content is 26%, but the derivatives of urea are more abundant Acid hydrolysis releases several amino acids Glycine is the most abundant (75% or more of total amino acid content), and its concentration considerably increases in some fractions when the hydrolysis is carried out at 130°C The role of free radicals in reactions leading to the formations of radiolytic products is considered Some comparisons are made between findings in the present work, at initial pH∼6, and an earlier study of ammonium cyanide at pH 9

Order and orientation of genic sequences in circular mitochondrial DNA from Saccharomyces exiguus: implications for evolution of yeast mtDNAs.

Abstract: Mapping of sequences specifying the large and small ribosomal RNAs and six polypeptides in the circular 23.7 kbp mitochondrial DNA ofSaccharomyces exiguus has shown that these genes have the same orientation and that a 5 gene cluster is common to this DNA and the 18.9 kbp mtDNA fromTorulopsis glabrata. Included in the preserved region are juxtaposed sequences specifying ATPase subunits 6 and 9 which have the same order and orientation as analogous genes in theEscherichia coli unc operon. The above data, together with knowledge that these two sequences are dispersed in larger yeast mtDNAs, leads us to suggest that larger forms are derived from a smaller ancestral molecule that would have had some resemblance to the mtDNAs ofS. exiguus andT. glabrata.

CpG frequency in large DNA segments

Abstract: The relationship between the deficiency of CpG dinucleotides and the coding-noncoding segments of DNA has been examined. Analysis of five human alpha-like globin DNA sequences and five human beta-like globin DNA sequences reveal that there is no apparent difference between protein coding and non-coding portions of DNA. Rather CpG deficiency appears to be a property of long contiguous segments of DNA consisting of several genes and their intergenic regions. Thus we propose that CpG deficiency is not involved with translation or transcription but rather is related to chromosomal constraints.

Evolution of the genome size in Akodon (Rodentia, Cricetidae)

Abstract: Nuclear DNA contents were estimated by microdensitometry in five species ofAkodon rodents:Adodon molinae, A. dolores, A. mollis, A. azarae, Bolomys obscurus) and in three chromosomal varieties ofA. molinae (2n=42; 2n=43, 2n=22). The data obtained showed that the species with the highest DNA content wasB. obscurus, followed in order of decreasing genome size byA. molinae, A. mollis, A. dolores andA. azarae. InA. molinae the forms with 2n=42 chromosomes had the lowest and the forms with 2n=44 the highest amount of DNA, while the forms with 2n=43 had intermediate DNA contents. The variation in DNA amount detected inA. molinae was interpreted as a phenomenon of amplification occurring in the chromosomal areas involved in the chromosomal rearrangement giving rise to the polymorphism exhibited by this species. The DNA contents of shared chromosomes (chromosomes with similar size, morphology and G banding pattern, which are found in two or more phylogenetically related species), were compared and correlated with values of total nuclear DNA. The information obtained indicates that: (a) shared chromosomes have variable amounts of DNA: (b) in a given species there is a correlation between the amount of nuclear and chromosomal DNA in most shared chromosomes (and perhaps in most of the chromosmal complement), e.g., the higher the amount of nuclear DNA, the higher the content of DNA in shared chromosomes; (c) some chromosomes may undergo processes of amplification or deletion restricted to certain regions and usually related with mechanisms of chromosomal rearrangements. The data obtained seem to indicate that the genome size of a species depends on the interaction of two independent mechanisms: (1) a general process of DNA variation which acts coincidentally in the same direction (gain or loss) in the whole chromosome complement without disturbing the mechanism of chromosome condensation producing G bands and (2) restricted processes of DNA variations (amplifications or deletions) which act on single chromosomes and which are in most cases related with chromosmal rearrangements.

Origin of the duplicated ribonuclease gene in guinea-pig: comparison of the amino acid sequences with those of two close relatives: capybara and cuis ribonuclease.

Abstract: The amino acid sequences of the pancreatic ribonuclease from capybara (Hydrochoerus hydrochaeris) and cuis (Galea musteloides) were determined. Both species belong to the same superfamily of the hystricomorph rodents as the guinea-pig. In guineapig pancreas two ribonucleases are present as a result of a rencent gene duplication, but in capybara and cuis pancreas only one single ribonuclease has been found. A most parsimonious tree of ribonucleases indicates that the gene duplication leading to both guinea-pig ribonucleases occurred before the divergence of guineapig from capybara and cuis. This would mean that changes in expression of the ribonuclease genes have occurred in these taxa.

Relationships among DNA sequences of the 1.3 kb EcoRI family of mouse DNA

Abstract: The genome of the mouse (Mus musculus) contains a family of repeated DNA sequences defined by a 1.3 kb EcoRI fragment. Resqtriction maps of ten cloned fragments from this family have been determined. The fragments were of seven different types, based on the patterns of digestion obtained with AvaII, HindIII, and TaqI restriction enzymes. These seven unique sets of sequences fell into two classes, as defined by the position of a single HindIII site. Portions of fragments from each of the two classes were sequenced. Although certain regions of the repeat were highly conserved between classes, there was more intraspecific sequence divergence among the sequenced regions than has been observed for the short interspersedAlu family of repeated sequences sin mammals. Sequences of both HindIII classes were found to be present within the mouse X chromosome; we can conclude that both classes must also be present on other mouse chromosomes.

Highly efficient peptide formation from N-acetylaminoacyl-AMP anhydride and free amino acid

Abstract: The kinetics of formation of the N-blocked dipeptide, N-acetylglycylglycine, from N-acetylglycyl adenylate anhydride and glycine in aqueous solution at 25 C, and at various PH's are reported. The reaction is of interest in that over a physiologically relevant pH range (6-8), peptide synthesis proceeds more rapidly than hydrolysis, even at those pH's at which this compound becomes increasingly susceptible to base-catalyzed hydrolysis. Under similar conditions, the corresponding unblocked aminoacyl adenylate anhydrides are considerably more unstable, and undergo appreciable hydrlysis in the presence of free amino acid. Because N-blocked aminoacyl adenylate anhydrides serve as model compounds of peptidyl adenylate anhydrides, these results suggest that primitive amino acid polymerization systems may have operated by cyclic reactivation of the peptidyl carboxyl group, rather than that of the incoming amino acid.