Showing papers on "Mutant published in 1976"

A mutant of Saccharomyces cerevisiae defective for nuclear fusion

Abstract: A mutant unable to fuse nuclei during mating has been isolated from standard wild-type Saccharomyces cerevisiae. Tetrad analysis of the mutation responsible for this defect (kar1-1) shows that it segregates as a single Mendelian factor. The defect kn kaf1-1 appears to be nuclear limited. Cytological and genetic evidence shows that in this mutant the events associated with zygote formation are normal until the point of nuclear fusion. The consequence of this defect is the formation of a multinucleate zygote which in subsequent divisions can segregate heterokaryons and haploid heterplasmons.

The enzymatic deficiency conditioned by the shrunken-1 mutations in maize.

Abstract: Evidence is presented to show that the Sh locus specifies sucrose synthetase in the developing endosperm of maize. The sh/sh/sh endosperm possesses less than 10% sucrose synthetase activity as compared to the normal Sh/sh/sh endosperm. The residual enzyme activity in five independently derived mutant genotypes is attributable to a protein molecule of different electrophoretic and immunochemical specificities that is presumably independent of the sh locus. Sucrose synthetase activity in the embryo in both the genotypes is electrophoretically indistinguishable from the one present in the mutant endosperm. Mutant endosperm has a reduced starch content as compared to the normal. This observation constitutes genetic evidence supporting a critical role for sucrose synthetase in starch biosynthesis.

Uv Mutagenesis in Radiation-Sensitive Strains of Yeast

Abstract: The yeast Saccharomyces cerevisiae appears to possess a single mutagenic or "error prone" pathway for the repair of UV damage, probably involving the functions of at least seven genes; rev1, rev2, rev3 ( Lemontt 1971a), rad6, rad8, rad9 and rad18 ( Lawrence et al. 1974; present results). Strains carrying rad6 are the most sensitive to the lethal effects of UV light in this group and double mutants carrying rad6 and either rev1, rev3, rad9 or rad18 are no more sensitive than this single mutant strain. rev3 rad6 double mutant diploids failed to show any UV-induced reversion of the normally highly revertible ochre allele cyc1–9 , even though a total of more than 2.5 x 109 viable cells was examined, suggesting that strains of this kind are entirely UV-immutable; spontaneous revertants could be recovered, however.—The rad6 and rev3 gene products would appear to be necessary for all kinds of mutagenic events at all sites within the genome, but the products of the other genes that act in the "errorprone" pathway hage a more restricted role and are involved in the production of only some kinds of mutations. It is suggested that such selectivity arises from the interaction of some repair enzymes with specific nucleotide sequences.

Genetic analysis of adenovirus type 2 III. Temperature sensitivity of processing viral proteins.

Abstract: Using sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis of [35S]methionine-labeled adenovirus type 2-infected KB cell extracts, a total of 23 virus-induced polypeptides was detected. This technique was applied to the analysis of the temperature-sensitive mutant, ts 1, which has previously been shown to be defective in a late function. By means of pulse-chase experiments, ts 1 was shown to be defective in the processing of the precursor polypeptide (Pre VII) to the major core protein VII. Two other putative precursor polypeptides, Va (27K) and Vb (24K), were also not processed. Thus, the ts 1 mutation blocked the appearance of six post-translational clevage products, i. e., polypeptides VI, VII, VIII, X, XI, and XII. All of these polypeptides are virion components. Processing was temperature sensitive in a shift-up experiment, whereas it was normal in a shift-down experiment. The kinetics of the temperature-shift experiments suggested that infectious virus could be recovered if enough time is provided for processing to take place. Processing was not inhibited by cycloheximide. The analysis of purified virus particles and empty shells (TCs) revealed the presence of the precursor and putative precursor polypeptides Pre-VII, Va and Vb, instead of their cleavage products, in both types of particles. Based on these results we propose that the ts 1 gene codes for or regulates an endoprotease which is responsible for the completion of the last step in virus maturation, that is, the conversion of "young virions" into mature infectious virions by a series of maturation cleavages.

Localization in yeast mitochondrial DNA of mutations expressed in a deficiency of cytochrome oxidase and/or coenzyme QH2-cytochrome c reductase.

Abstract: 1 Three methods are described for the genetic analysis of yeast cytoplasmic mutants (mit−mutants) lacking cytochrome oxidase or coenzyme QH2-cytochrome c reductase. The procedures permit mutations in mitochondrial DNA to be mapped relative to each other and with respect to drug-resistant markers. The first method is based upon the finding that crosses of mit− mutants with some but not other isonuclear Q− mutants lead to the restoration of respiratory functions. Thus a segment of mitochondrial DNA corresponding to a given mit− mutation or to a set of mutations can be delineated. The second method is based on the appearance of wild-type progeny in mit−× mit+ crosses. The third one is based on the analysis of various recombinant classes issued from crosses between mit−, drug-sensitive and mit+, drug-resistant mutants. Representative genetic markers of the RIBI, OLII, OLI2 and PARI loci were used for this purpose. 2 The three methods when applied to the study of 48 mit− mutants gave coherent results. At least three distinct regions on mitochondrial DNA in which mutations cause loss of functional cytochrome oxidase have been established. A fourth region represented by closely clustered mutants lacking coenzyme QH2-cytochrome c reductase and spectrally detectable cytochrome b has also been studied. 3 The three genetic regions of cytochrome oxidase and the cytochrome b region were localized by the third method on the circular map, in spans of mitochondrial DNA defined by the drugresistant markers. The results obtained by this method were confirmed by analysis of the crosses between selected mit− mutants and a large number of Q− clones whose retained segments of mitochondrial DNA contained various combinations of drug-resistant markers. 4 All the genetic data indicate that the various regions studied are dispersed on the mitochondrial genome and in some instances regions or clusters of closely linked mutations involved in the same respiratory function (cytochrome oxidase) are separated by other regions which code for entirely different functions such as ribosomal RNA.

Control of protein synthesis in herpesvirus-infected cells: analysis of the polypeptides induced by wild type and sixteen temperature-sensitive mutants of HSV strain 17.

Abstract: Summary The polypeptides induced in cells infected with a Glasgow isolate of HSV-1 (17 syn +) have been characterized by SDS polyacrylamide gel electrophoresis. Study of the kinetics of synthesis in three cell lines has detected a total of 52 polypeptides, 33 of which can be identified in polypeptide profiles of purified virions. These include six low mol. wt. polypeptides that have not been previously reported. Several polypeptides were labelled with glucosamine in infected BHK cells. The different polypeptide patterns obtained at permissive (31 °C) and non-permissive (38 °C) temperature in cells infected with 16 temperature-sensitive (ts) mutants are reported. The effect of multiplicity of infection (m.o.i.) on the polypeptide profile has been examined for two of the DNA — ve mutants: below ten, the profile varied with the m.o.i. whereas above ten it was constant. All mutants were therefore examined at an m.o.i. of approx. 20. Mutants from the same complementation group showed very similar profiles. A number of general conclusions concerning control of protein synthesis in HSV infected cells can be made: (1) As most of the 16 ts mutants affected the synthesis of several or many polypeptides it follows that a large proportion of the genome specifies controlling functions. (2) The high frequency with which some polypeptides were affected suggests they are at or near the terminus of biosynthetic pathways which are under multiple control. (3) Conversely, some polypeptides were affected with a low frequency suggesting that their synthesis is not dependent on the expression of many virus functions. (4) Several individual ts mutations lead to the synthesis of increased amounts of different large polypeptides. (5) Analysis of every band detectably affected by at least one ts mutation has disclosed nine classes of dependence relationship between polypeptide synthesis and the DNA phenotype of the mutants, illustrating that this relationship is complex and different for different polypeptides. (6) The inhibition of host protein synthesis by the virus may not be a simple single step process.

Autolytic enzyme-deficient mutants of Bacillus subtilis 168.

Abstract: Mutants of Bacillus subtilis strain 168 have been isolated that are at least 90 to 95% deficient in the autolytic enzymes N-acetylmuramyl-L-alanine amidase and endo-beta-N-acetylglucosaminidase. These mutants grow at normal rates as very long chains of unseparated cells. The length of the chains is directly related to the growth rates. They are nonmotile and have no flagella, but otherwise appear to have normal cell morphology. Their walls are fully sysceptible to enzymes formed by the wild type and have the same chemical composition as the latter. Cell wall preparations from the mutants lyse at about 10% of the rate of those from the isogenic wild type, with the correspondingly small liberation of both the amino groups of alanine at pH 8.0 and of reducing groups at pH 5.6. Likewise, Microcococcus luteus walls at pH 5.6 and B. subtilis walls at pH 8 are lysed only very slowly by LiCl extracts made from the mutants as compared with rates obtained with wild-type extracts. Thus, the activity of both autolytic enzymes in the mutants is depressed. The frequencies of transformation, the isolation of revertants, and observations with a temperature-sensitive mutant all point to the likelihood that the pleiotropic, phenotypic properties of the strains are due to a single mutation. The mutants did not produce more protease or amylase than did the wild type. They sporulate and the spores germinate normally. The addition of antibiotics to exponentially growing cultures prevents wall synthesis but leads to less lysis than is obtained with the wild type. The bacteriophage PBSX can be induced in the mutants by treatment with mitomycin C.

Autoregulation of simian virus 40 gene A by T antigen.

Abstract: During lytic infection by simian virus 40, gene A is transcribed into early RNA, which is translated into A protein (T antigen). Both the rate of synthesis and the intracellular amount of early RNA are higher in cells infected by temperature-sensitive A (tsA) mutants than in cells infected by wild-type virus. These differences are observed at permissive temperature (32 degrees) and are amplified greatly after a shift to restrictive temperature (41 degrees). For example, at 32 degrees cells infected by tsA mutants synthesize early RNA approximately twice as fast as cells infected by wild-type virus. After the shift to 41 degrees, the rate of synthesis in the tsA infection increases to 15 times the rate in the wild-type infection. In contrast, cells infected by tsA mutants do not overproduce late RNA. We suggest that the A protein regulates its own synthesis by negative feedback control of gene A transcription.

Isolation and characterization of conditional lethal mutants of Escherichia coli defective in transcription termination factor rho.

Abstract: Polarity suppressor mutants that are conditional lethal for growth have been isolated in E. coli K12. The mutations map between the ilv and cya loci of the E. coli chromosome. Rho factor isolated from one of these ts mutants does not show transcription termination activity at any temperature tested; however, it is found to be temperature sensitive for its poly(C)-dependent ATPase activity. Unlike the previously known polarity suppressor mutants (suA and psu), the rho mutation suppresses all types of polarity. Other interesting properties of these mutants include ultraviolet sensitivity, recombination deficiency, and decreased ability to lysogenize temperate phages lambda and P1. Our results suggest that rho has an essential function in the growth and normal physiology of cells. The rho(ts) mutant allows the growth of phage lambda defective in the N gene. This result supports the model that N gene product prevents transcription termination by antagonizing rho activity.

THE mei-9a MUTANT OF DROSOPHILA MELANOGASTER INCREASES MUTAGEN SENSITIVITY AND DECREASES EXCISION REPAIR

Abstract: The mei-9a mutant of Drosophila melanogaster , which reduces meiotic recombination in females (Baker and Carpenter 1972), is deficient in the excision of UV-induced pyrimidine dimers in both sexes. Assays were performed in primary cultures and established cell lines derived from embryos. An endonuclease preparation from M. luteus , which is specific for pyrimidine dimers, was employed to monitor UV-induced dimers in cellular DNA. The rate of disappearance of endonuclease-sensitive sites from DNA of control cells is 10–20 times faster than that from mei-9a cells. The mutant mei-218, which is also deficient in meiotic recombination, removes nuclease-sensitive sites at control rates. The mei-9a cells exhibit control levels of photorepair, postreplication repair and repair of single strand breaks. In mei-9 cells DNA synthesis and possibly postreplication repair are weakly sensitive to caffeine. Larvae which are hemizygous for either of the two mutants that define the mei-9 locus are hypersensitive to killing by the mutagens methyl methanesulfonate, nitrogen mustard and 2-acetylaminofluorene. Larvae hemizygous for the mei-218 mutant are insensitive to each of these reagents. These data demonstrate that the mei-9 locus is active in DNA repair of somatic cells. Thus functions involved in meiotic recombination are also active in DNA repair in this higher eukaryote. The results are consistent with the earlier suggestions (Baker and Carpenter 1972; Carpenter and Sandler 1974) that the mei-9 locus functions in the exchange events of meiosis. The mei-218 mutation behaves differently in genetic tests and our data suggest its function may be restricted to meiosis. These studies demonstrate that currently recognized modes of DNA repair can be efficiently detected in primary cell cultures derived from Drosophila embryos.

Characterization of postreplication repair in mutagen-sensitive strains of drosophila melanogaster

Abstract: Mutants of Drosophila melanogaster , with suspected repair deficiencies, were analyzed for their capacity to repair damage induced by X-rays and UV radiation. Analysis was performed on cell cultures derived from embryos of homozygous mutant stocks. Postreplication repair following UV radiation has been analyzed in mutant stocks derived from a total of ten complementation groups. Cultures were irradiated, pulse-labeled, and incubated in the dark prior to analysis by alkaline sucrose gradient centrifugation. Kinetics of the molecular weight increase in newly synthesized DNA were assayed after cells had been incubated in the presence or absence of caffeine. Two separate pathways of postreplication repair have been tentatively identified by mutants derived from four complementation groups. The proposed caffeine sensitive pathway (CAS) is defined by mutants which also disrupt meiosis. The second pathway (CIS) is caffeine insensitive and is not yet associated with meiotic functions. All mutants deficient in postreplication repair are also sensitive to nitrogen mustard. The mutants investigated display a normal capacity to repair single-strand breaks induced in DNA by X-rays, although two may possess a reduced capacity to repair damage caused by localized incorporation of high specific activity thymidine- 3 H. The data have been employed to construct a model for repair of UV-induced damage in Drosophila DNA. Implications of the model for DNA repair in mammals are discussed.

Isolation of nonsense suppressor mutants in Pseudomonas.

Abstract: A strain of Escherichia coli harboring the drug resistance plasmid RP1 was treated with the mutagen N-methyl-N-nitro-N-nitro-N-nitrosoguanidine, and mutants were isolated in which ampicillin resistance had been lost due to an amber mutation in the plasmid. One of these mutants was again treated, and a strain was isolated in which tetracycline resistance was also lost due to an amber mutation in the plasmid. The plasmid containing amber mutations in the genes amp and tet was named pLM2. This plasmid could be transferred to strains of Pseudomonas aeruginosa, P. phaseolicola, and P. pseudoalcaligenes. Mutants resistant to ampicillin and tetracycline could not be obtained from P. phaseolicola carrying pLM2. However, strains of E. coli, P. aeruginosa, and P. pseudoalcaligenes carrying the plasmid did produce mutants simultaneously resistant to both antibiotics. All of the mutants of E. coli had developed nonsense suppressors since they became phenotypically lac+, although harboring a lac amber mutation, and formed plaques with amber mutants of phages PRR1 and PRD1 that attack organisms carrying RP1. Approximately 20% of the resistant mutants of P. aeruginosa and P. pseudoalcaligenes were sensitive to the amber mutant of PRD1. These mutants were of variable stability and grew somewhat more slowly than their parent strains. One of the suppressor mutants of P. pseudoalcaligenes, designated ERA(pLM2)S4, was used for the isolation of nonsense mutants of bacteriophage PHA6, a virus having a segmented genome of double-stranded ribonucleic acid and an envelope of lipids and proteins.

Energy metabolism in respiration-deficient and wild type Chinese hamster fibroblasts in culture.

Abstract: This paper presents a comparison of energy metabolism in wild type and respiration-deficient Chinese hamster cells. From previous work (DeFrancesco et. al., '75) it was concluded that the mutant satisfies essentially all of its energy requirements from glycolysis and in this study we measure precisely the amount of glucose consumed and lactate produced per milligram increment of protein in exponentially growing cultures. From these measurements we calculate the amount of ATP derived from glycolysis (and hence the total energy requirement for normal proliferation) to be 105 +/- 15 mumoles ATP/delta mg protein in the mutant. It is 63 +/- 10 mumoles ATP/delta mg protein derived from glycolysis in wild type cells. We present evidence that the total energy requirement of wild type cells is similar to that of the mutant suggesting that approximately 40% of the energy requirement is derived from respiration. The oxidation of glutamine appears to be more significant than the complete oxidation of glucose to CO2 in these Chinese hamster fibroblasts. The amount of ATP required by the mutant cells per milligram increment of protein is relatively independent of pH.

Localization of gene functions in polyoma virus DNA

Abstract: Polyoma virus mutants of four functionally distinct groups have been mapped by the marker rescue technique using restriction enzyme fragments of wild-type viral DNA. Nontransforming host-range mutants map in the proximal part of the early region of the viral genome. The same DNA fragment that restores a normal host range also restores normal transforming ability to these mutants. ts-25D, a temperature-sensitive (ts)-a class mutant, maps in the distal part of the early region. ts-3 and ts-1260 map in the proximal and distal parts of the late region, respectively.

Genetic controls of meiotic recombination and somatic DNA metabolism in Drosophila melanogaster.

Abstract: Recombination-defective meiotic mutants and mutagen-sensitive mutants of D. melanogaster have been examined for their effects on meiotic chromosome behavior, sensitivity to killing by mutagens, somatic chromosome integrity, and DNA repair processes. Several loci have been identified that specify functions that are necessary for both meiotic recombination and DNA repair processes, whereas mutants at combination and DNA repair processes, whereas mutants at other loci appear to be defective in only one pathway of DNA processing.

Two chromosomal genes required for killing expression in killer strains of Saccharomyces cerevisiae.

Abstract: The killer character of yeast is determined by a 1.4 x 106 molecular weight double-stranded RNA plasmid and at least 12 chromosomal genes. Wild-type strains of yeast that carry this plasmid (killers) secrete a toxin which is lethal only to strains not carrying this plasmid (sensitives). ——— We have isolated 28 independent recessive chromosomal mutants of a killer strain that have lost the ability to secrete an active toxin but remain resistant to the effects of the toxin and continue to carry the complete cytoplasmic killer genome. These mutants define two complementation groups, kex1 and kex2. Kex1 is located on chromosome VII between ade5 and lys5. Kex2 is located on chromosome XIV, but it does not show meiotic linkage to any gene previously located on this chromosome. ——— When the killer plasmid of kex1 or kex2 strains is eliminated by curing with heat or cycloheximide, the strains become sensitive to killing. The mutant phenotype reappears among the meiotic segregants in a cross with a normal killer. Thus, the kex phenotype does not require an alteration of the killer plasmid. ——— Kex1 and kex2 strains each contain near-normal levels of the 1.4 x 106 molecular weight double-stranded RNA, whose presence is correlated with the presence of the killer genome.

SAMase gene of bacteriophage T3 is responsible for overcoming host restriction.

Abstract: Deletion and point mutants of T3 have been isolated and used to show that the early region of T3 DNA is organized in the same way as that of T7 DNA Homologous early RNAs and proteins of the two phages have been identified by electrophoresis on polyacrylamide gels in the presence of sodium dodecyl sulfate Both phages have five early mRNA's, numbered 03, 07, 1,11 and 13 from left to right, although no T3 protein that corresponds to the 11 protein of T7 has yet been identified In general, corresponding early RNAs and proteins of the two phages migrate differently on gels, indicating that they differ in molecular weight and/or conformation In both T7 and T3, gene 03 is responsible for overcoming the DNA restriction system of the host, gene 07 specifies a protein kinase, gene 1 specifies a phage-specific RNA polymerase, and gene 13 specifies a polynucleotide ligase The 03 protein of T3 is responsible for the S-adenosylmethionine cleaving activity (SAMase) induced after T3 (but not T7) infection However, cleaving of S-adenosylmethionine does not appear to be the primary mechanism by which T3 overcomes host restriction, since at least one mutant of T3 has lost the SAMase activity without losing the ability to overcome host restriction

Genetic regulation of storage protein content in maize endosperm

Abstract: Sodium dodecylsulfate-polyacrylamide gel electrophoresis reveals that zein prepared from normal maize inbred (Zea mays L.) contains six separable components. Z1 and Z2 are the predominant species, with molecular weights of 21,800 and 19,000 daltons. Amino acid analysis of these two components shows that both are rich in glutamic acid, leucine, and proline, but low in lysine. Of the four minor bands, Z3, Z4, Z5, and Z6, the latter two exist only in trace amounts. A mutation at the opaque-2 locus severely suppresses the synthesis of Z1. The nonallelic mutant, opaque-7, strongly suppresses the synthesis of Z3 and Z4, while slightly reducing Z2. On the other hand, the floury-2 mutant appears to reduce the synthesis of these six proteins in the same relative proportion. In the double mutant combinations, opaque-2 apparently is epistatic to opaque-7 and floury-2 in the synthesis of zein components. The glutelin fraction shows a more complex banding pattern; however, qualitative differences are not apparent among the mutant lines examined.

A chromosomal gene required for killer plasmid expression, mating, and spore maturation in Saccharomyces cerevisiae.

Abstract: "Killer" strains of Saccharomyces cerevisiae are those that harbor a double-stranded RNA plasmid and secrete a toxin that kills only strains not carrying this plasmid (sensitives). Two chromosomal genes (kex1 and kex2) are required for the secretion of toxin by plasmid-carrying strains. The kex2 gene, which maps at a site distinct from the mating-type locus, is also required for normal mating by alpha strains and meiotic sporulation in all strains. Strains that are alpha mating-type and kex2 fail to secrete the pheromone alpha-factor or to respond to the alpha-factor II pheromone which causes a morphological change, but they do respond to alpha-factor I which causes G1 arrest in alpha cells. Strains that are alpha mating-type and kex2 show no defect in mating; pheromone secretion, or response to alpha-factor. Diploids that are homozygous for the kex2 mutation, unlike wildtype or heterozygous diploids, fail to undergo sporulation, with the defect occurring in the final spore maturation stage. These same defects in the sexual cycle are present in all kex2 mutants independent of the presence of the "killer" plasmid.

H-2 compatibility requirement for virus-specific T-cell-mediated cytolysis. The H-2K structure involved is coded by a single cistron defined by H-2Kb mutant mice.

Abstract: Lymphocytic choriomeningitis or vaccinia virus-immune spleen cells of H-2 mutant mice carrying a point mutation in the K region (B6 H-2ba, B6 H-2bf) cannot lyse infected wild-type H-2Kb targets and vice versa. Yet, cytotoxic T cells specific for infected H-2Kba or H-2Kbf targets are generated during virus infections as shown by cold target competition experiments. The critical structure for the apparent restriction by the K or D regions of the H-2 gene complex of cytolytic interactions between T cells and virus-infected target cells are therefore each coded, at least as shown for the K region, by a single cistron. This finding is most readily accommodated within the altered self concept (postulating that T cells are specific for virus-modified self structures) but cannot exclude the possibility of a physiological interaction mechanism being responsible for the apparent H-2 restriction of virus-specific cytotoxic T cells.

Synaptic transmission reversibly conditioned by single-gene mutation in Drosophila melanogaster.

Abstract: ONE way to advance current physiological and biochemical understanding of the mechanism of synaptic transmission at the molecular level is to alter synaptic function by means of single gene mutations. Among the various behavioural mutants of Drosophila melanogaster, those which express a mutant phenotype only in certain conditions and otherwise behave normally are of particular interest. If the mutant phenotype appears at the synapse, the mutant might enable a reversible modification of synaptic function by way of experimentally controlling the expression of the mutant phenotype. Such a mutant would provide a useful tool for the study of synaptic function.

Mutants of herpes simplex virus types 1 and 2 that are resistant to phosphonoacetic acid induce altered DNA polymerase activities in infected cells.

Abstract: Summary Three mutants of herpes simplex virus (HSV) have been isolated which form plaques in the presence of 100 µg/ml phosphonacetic acid (PPA). All three mutants (3 from HSV-1 strain 17 syn +, 14 from HSV-1 strain 17 syn, and 19 from HSV-2) induce viral DNA synthesis and viral DNA polymerase activity, and these are much less sensitive to PPA than the wild-type virus. The results support the hypothesis that PPA interacts directly with the viral DNA polymerase protein, at least part of which is virus coded.

Bacteriophage-resistant mutants of Salmonella typhimurium deficient in two major outer membrane proteins.

Abstract: Mutants resistant to bacteriophages (P221 and PH105 or PH51) were isolated from a rfa strain of Salmonella typhimurium. They were found deficient in separate 33,000- to 36,000-dalton band proteins (major band proteins). Double mutants derived from both types of mutants were deficient in both of the bands. The growth behavior of all the mutants was normal. The outer membrane of the mutants appeared to be more wrinkled than normal and formed vesicles in many of the mutants. In freeze-fractured cells, changes were seen in the outer membrane (particleless patches in the concave fracture face, the particles themselves being smaller than normal). These changes were more marked in the double mutants.

Iron uptake in colicin B-resistant mutants of Escherichia coli K-12.

Abstract: Four classes of colicin B-resistant mutants of Escherichia coli K-12 were examined for defects in iron uptake. All four mutant classes (cbt, exbC, exbB, and tonB) were defective in the uptake of ferri-ennterochelin. The tonB mutant was also defective in citrate-, ferrichrome-, and rhodoturulic acid-mediated iron uptake. The defects in iron transport were reflected in increased sensitivity to iron chelators and to chromium and aluminium salts, and in hypersecretion of enterochelin. One of the mutants (cbt) was apparently defective in outer membrane ferri-enterochelin receptor activity. aroE derivatives (unable to synthesize enterochelin) of the four mutant classes and the parent strain produced increased amounts of two outer membranes polypeptides when grown under iron stress. These polypeptides are implicated in ferri-enterochelin receptor activity.