Showing papers in "Molecular Genetics and Genomics in 1970"

Escherichia coli mutants with temperature-sensitive synthesis of DNA.

Abstract: Seven mutants of E. coli with temperature-sensitive synthesis of DNA have been isolated. Synthesis of RNA, protein and DNA precursors does not appear to be directly affected. The mutants can be divided into at least two groups on the basis of their pattern of DNA synthesis, their ability to support phage growth at 41° and their genetic mapping. Mutants of the first group are heterogeneous in their pattern of DNA synthesis at 40°. Some mutants cease DNA synthesis abruptly upon transfer to 40° and any residual DNA synthesis is barely detectable. In others there is substantial residual synthesis at 40°. All these Group 1 mutants are alike, however, in that they support the growth of phage T4 but not Lambda at 41°. Two mutants with barely detectable residual DNA synthesis carry DNA mutations which have been mapped by P1 transduction and show about 72% linkage to the malB locus. It has not yet proved possible to map accurately the mutants showing substantial residual synthesis, and the possibility that these mutations are in a different gene(s) has not been excluded. A single mutant has been placed in a second group. Like some Group 1 mutants it synthesizes substantial amounts of DNA at 40° before synthesis stops. However, unlike them it supports the growth of T4 and Lambda at 41°. The DNA mutation maps near the leu locus. Certain properties of this mutant are consistent with the idea that initiation of DNA synthesis is temperature-sensitive in this strain.

Isolation and genetic analysis of temperature-sensitive mutants of B. subtilis defective in DNA synthesis.

Abstract: Fifty temperature-sensitive mutants defective in DNA synthesis at high temperature have been identified among 655 temperature-sensitive mutants isolated at random from a mutagenised population of B. subtilis. They are distributed in a non-random fashion in 9 genetic linkage groups, located in different regions of the B. subtilis genome. It is suggested that at least 14 genes are involved in B. subtilis DNA replication.

Evidence for genetic transformation in blue-green alga Anacystis nidulans.

Abstract: Evidence has been presented that blue-green alga Anacystic nidulans can undergo genetic transformation DNA from erythromycin-, streptomycin-resistant of filamentous strains has been found to transform appropriate markers to a wild type or some other recipients Favourable conditions for transformation have been described with respect to the revealing of transformants, the concentration of DNA and the competence of cells

Host specificity of DNA produced by Escherichia coli. XII. The two restriction and modification systems of strain 15T

Abstract: SummaryE. coli 15T- carries two distinct sets of DNA restriction and modification activities. The genetic information for system A is contained in the bacterial chromosome and linked to the thr region. This fact suggests host specificity A to be related to those of strains K and B. The genes controlling system 15 are on a plasmid which is related to phage Pl: it competes with Pl for stable inheritance in the carried state and it genetically recombines with Pl. This recombination may produce plasmid genomes with newly assorted characters (see Table 3). One of them is an active, Pl-like prophage with the 15-specific instead of the parental Pl-specific restriction and modification characters. Superinfection of 15T- with Pl may also result in curing of the bacteria from the restriction plasmid.Bot A- and 15-specific restrictions and modifications act on bacterial DNA, on the DNA of various sex factors and on the DNA of certain bacteriophages, e.g. of phage λ. Phage 82 DNA is sensitive only to 15-specific restriction, but not to A-specific restriction.Independently of the A- and 15-specific restrictions, the growth of phage λ in E. coli 15T- encounters another limitation of yet unknown nature. No such limitation is observed either with phage 82 or with mutants of λ occurring at a frequency of about 10-5.

Identification of ten genes that control ribosome formation in yeast.

Abstract: Twenty-three temperature-sensitive mutants of Saccharomyces cerevisiae, all of which undergo a rapid cessation of net RNA accumulation following a shift from the permissive (23°) to the restrictive temperature (36°), have been characterized. Genetic studies demonstrate that these mutants belong to ten different complementation groups and that, in most cases, their properties are the result of a single, recessive mutation in a nuclear gene. Although the mutants were isolated for heat sensitivity, mutants from 2 of the complementation groups are cold sensitive (at 13°) as well. The mutants continue to synthesize protein, including an enzyme, alkaline phosphatase, for two to four hours following a shift from 23° to 36°, suggesting that they are capable of messenger RNA synthesis and the translation of messenger RNA with fidelity at the restrictive temperature. The small amount of RNA that is synthesized in these mutants at the restrictive temperature has been examined on sucrose gradients and by acrylmide gel electrophoresis; in addition, the RNA components in polyribosomes have been fractionated by a new technique that separates messenger RNA from ribosomal RNA. As a result of these analyses we conclude that these mutants are strongly inhibited in the accumulation of 5S, 7S, 17S, and 25S RNA components but are only slight if at all inhibited in the synthesis of messenger RNA and 4S RNA. The results reported here define ten genes, designated rna 2 through rna 11, that play an essential role in the formation or maturation of ribosomes in yeast.

Molecular interactions of ribosomal components. I. Identification of RNA binding sites for individual 30S ribosomal proteins.

Abstract: The formation of a complex between individual 30S ribosomal proteins and 16S ribosomal RNA was studied by three techniques: zone centrifugation, molecular-sieve chromatography and electrophoresis in polyacrylamide gels. Five 30S proteins form a stable complex with the RNA under the conditions used to assemble ribosomes. Specific and nonspecific complex formation can be distinguished by an analysis of the concentration-dependence for complex formation. Similarly, competition experiments between heterologous proteins that bind to RNA can also be used to establish the uniquness of the RNA binding sites for ribosomal proteins. The data show that four of the five proteins bind to unique sites on the RNA. The fifth protein binds nonspecifically to the RNA. In addition, cooperative interactions between several proteins were observed; these enhance the interaction of proteins with the 16S RNA. A partial assembly sequence for the 30S ribosomal subunit is presented.

Genetic and transfection studies with B, subtilis phage SP 50. I. Phage mutants with restricted growth on B. subtilis strain 168.

Abstract: Summary“Functional” mutants of phage SP50 are described whose growth, in contrast to wild type SP50, is restricted on B. subtilis strain 168. On the basis of complementation tests and genetic crosses, these mutations could be grouped into three genes. Three types of mutants of strain 168, which permitted the growth of any one of the three classes of phage mutants were isolated.In transfection of 168 with SP50 wild type DNA the number of infective centers varies with the third to fourth power of the DNA concentration. Infection of competent 168 cells with an intact functional-mutant phage concomitant with transfection with wild type DNA greatly enhances the specific activity of such DNA and leads to a linear dependence between the number of infected centers and DNA concentration.

Ribosomal proteins. XV. Amino acid compositions of isolated ribosomal proteins from 30S and 50S subunits of Escherichia coli.

Abstract: SummaryIsolated ribosomal proteins ofE. coli were hydrolyzed and their amino acid compositions determined. The compositions (in moles per cent) are given for 30S proteins in Table 1 and for 50S proteins in Table 2. The data show that all proteins (except L7 and L12) are distinctly different from each other in respect to their amino acid compositions.

Genetic map of Escherichia coli strain C

Abstract: A genetic map for strain C of Escherichia coli was constructed, independently of that available for strain K-12 (Fig.2). The two maps are very similar. The origins of several Hfr derivatives of E. coli C were mapped. An F′ strain transferring the genes for histidine synthesis was isolated.

The genetic analysis of regulation of amidase synthesis in Aspergillus nidulans. I. Mutants able to utilize acrylamide.

Abstract: Aspergillus nidulans uses an acetamidase enzyme to grow on acetamide as a carbon or as a nitrogen source Acrylamide is a substrate for the enzyme but does not induce its synthesis Mutants capable of growing on acrylamide as a nitrogen source have been isolated Two classes of mutant have been found —amdRc mutants on linkage group II andamdTc on linkage group IIIamdRc mutants produce high constitutive acetamidase levels The enzyme is still inducible by amides, but to a lesser extent than wild type, and is still subject to repression by ammonia and by carbon metabolites derived from glucoseamdRc mutants are semi-dominant to the wild type allele in heterozygous, diploids TheamdTc mutant is not subject to carbon metabolite repression, of the acetamidase The enzyme is inducible by amides and repressible by ammonia TheamdTc mutation also results in reduced ability to grow on formamide as a nitrogen source and to lowered levels of a second amidase enzymeamdTc is semi-dominant in heterozygous diploids

One way to do experiments on gene conversion? Transfection with heteroduplex SPP1 DNA.

Abstract: Competent cells of B. subtilis were transfected with heteroduplex SPP1 DNA, made by annealing complementary strands of wild type and 21 plaque type mutant DNAs. The frequencies of cells yielding mutant and wild type, only wild type, and only mutant phages were determined by single burst analyses of transfected cells. The data obtained reveal that an effective mechanism is operating in B. subtilis, which converts heterozygous to homozygous molecules prior to their replication. This “correction” mechanism is asymmetric with regard to the strand which is preferentially corrected in a given heteroduplex pair. The direction of asymmetry thus defined depends on the marker introduced into a particular heteroduplex. The efficiency of correction varies with the markers used and is correlated to the position of markers in the genetic map. From this correlation, the direction of replication of the SPP1 genome is deduced. The frequency distribution of wild type and mutant phages in cells yielding both genotypes indicates that both strands of the input DNA contribute equally to the production of progeny, i.e. DNA replication is symmetric.

Functional heterogeneity of the 30S ribosomal subunit of E. coli

Abstract: When 30S ribosomal subunits from E. coli are incubated with poly U, two separable components are recovered by zonal centrifugation of the incubation mixture. The faster sedimenting component is an aggregate of 30S subunits and poly U, while the slower one corresponds to the 30S ribosomal subunit. One ribosomal protein, protein 30S-1 is predominantly present in the faster sedimenting aggregate. The amount of poly U-30S subunit complex formed in the incubation mixture is limited by the amount of protein 30S-1 present. Consequently the number of ribosomal binding sites available for Phe-tRNA is limited in a similar fashion by the presence of protein 30S-1. When 30S ribosomal subunits are reconstituted in the absence of protein 30S-1, very little poly U or Phe-tRNA binding capacity is manifest under our assay conditions. We conclude that protein 30S-1 is required for maximum capacity of ribosomes to bind mRNA. Since this protein is present only on a fraction of the ribosome at any one time, it must exchange from one ribosome to another during protein synthesis.

Reversion of a streptomycin-dependent strain of Escherichia coli.

Abstract: A streptomycin dependent, spectinomycin resistant mutant ofEscherichia coli was used to select spontaneous phenotypic revertants to non-dependence on streptomycin. The ribosomes from one such revertant, which is inhibited by both streptomycin and spectinomycin, were analyzedin vitro. The altered protein responsible for the suppression of the streptomycin dependent phenotype was identified; this protein is 30S-10. The genetic locus for this mutation is a newly identified locus and it has been positioned close to thestr locus. The identification of the altered component responsible for the suppression of the spectinomycin resistant phenotype may be the same as that for the streptomycin dependent phenotype, but this is unproven.

The genetic analysis of regulation of amidase synthesis in Aspergillus nidulans. II. Mutants resistant to fluoroacetamide.

Abstract: Aspergillus nidulans uses an acetamidase enzyme to grow on acetamide as a carbon or as a nitrogen source. Acrylamide is a substrate for the enzyme but does not induce its synthesis. Mutants capable of growing on acrylamide as a nitrogen source have been isolated. Two classes of mutant have been found —amdRc mutants on linkage group II andamdTc on linkage group III.amdRc mutants produce high constitutive acetamidase levels. The enzyme is still inducible by amides, but to a lesser extent than wild type, and is still subject to repression by ammonia and by carbon metabolites derived from glucose.amdRc mutants are semi-dominant to the wild type allele in heterozygous, diploids. TheamdTc mutant is not subject to carbon metabolite repression, of the acetamidase. The enzyme is inducible by amides and repressible by ammonia. TheamdTc mutation also results in reduced ability to grow on formamide as a nitrogen source and to lowered levels of a second amidase enzyme.amdTc is semi-dominant in heterozygous diploids.

Mutations of coliphage λ affecting the expression of replicative functions O and P

Abstract: A selection technique, using the thermoinducible prophage λCI857Nsus7 Nsus53, has lead to the characterization of a new class of prophage mutations (called r), which prevent host killing upon thermal induction

Ribosomal proteins. XVI. Altered S4 proteins in Escherichia coli revertants from streptomycin dependence to independence.

Abstract: SummaryThirteen phenotypic revertants from streptomycin dependence were independently isolated. The ribosomal proteins from these mutants were extracted and analyzed by two-dimensional polyacrylamide gel electrophoresis and by immunological methods. In four of these mutants examined, alterations in protein S4 were detected by a change in electrophoretic mobility. Three additional mutants have S4 proteins which are indistinguishable from the wild type in two-dimensional electrophoresis but which are clearly different in their immunological properties. Further analysis should reveal whether suppression of the streptomycin independent phenotype is always caused by an alteration in protein S4.

An explanation of fine structure map expansion in terms of excision repair

Abstract: Fine structure map expansion is a marker effect which can be explained on the basis of the repair of mismatched bases in hybrid DNA. The chance of a mismatched base pair being corrected independently of a closely linked mismatched pair will sharply increase as the distance between the two sites becomes greater than the length of the DNA segment involved in the correction process. The consequences of this explanation are worked out and it is shown that, if it is true, the mapping curve should show three phases: an initial additive phase when the recombining sites are closely linked, a phase of increased slope—map expansion-and a final additive phase of reduced slope beyond the expansion region. Comparison of the initial and final slopes should yield information on the relation between gene conversion and crossing-over. Many sets of experimental data show a clear transition from the initial additive region to that of map expansion, but evidence for the predicted final phase is scanty, possibly because fine structure maps cover too small a region of the chromosome. Using data from genes with known products, estimates can be made of the minimum length of the DNA segments involved in correction. These are calculated as about 40 nucleotides in fission yeast and at least 130 nucleotides in Neurospora.

Genetic effects of N-methyl-N'-nitro-N-nitrosoguanidine in Neurospora crassa.

Abstract: The mutagenicity of N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) was studied with a genetically marked, balanced heterokaryon of Neurospora crassa. Types of genetic alterations detectable in this system are (a) point mutations in the ad-3 A and ad-3 B loci, (b) multilocus (chromosome) deletions in the ad-3 region, and (c) recessive lethal mutations in the whole genome. Study of the inactivation kinetics of the heterokaryotic and homokaryotic conidial fractions makes it possible to distinguish between nuclear and cytoplasmic inactivation.

Relations physiologiques entre les phages tempérés λ et Φ80

Abstract: This work deals with the ability of phage Φ80 to provide defective mutants of λ with their missing functions. Functions Involved in Recombination. As shown by others, the Int mechanism of Φ80 cannot excise prophage λ. However, Φ80 efficiently excises recombinants from tandem dilysogens, using its Ter mechanism. Likewise, the nonspecific mechanism Red is interchangeable between Φ80 and λ. Maturation of λ DNA by Φ80. The “Ter recombinants” excised by Φ80 from tandem dilysogens are packaged into a Φ80 protein coat. This contrasts with the fact, already mentionned by Dove, that Φ80 is extremely inefficient for packaging phage λ superinfecting a λ-lysogen. The latter result is also found when the helper phage is a hybrid with the left arm of λ (Φ80hy4 or Φ80hy41 — see Fig. 1). However, the maturation of the superinfecting λ is much more efficient if the Φ80hy used as a helper has the att-N region of λ (like Φ80hy1). Conversely a λ with the att-N region of Φ80 (λhy6 — see Fig. 1) is packaged more efficiently by Φ80 or Φ80hy4 than by Φ80hy1. It is suggested that the maturation of λ chromosome superinfecting an immune cell requires a recombination with the helper phage. Vegetative Functions. Among the replicative functoons O and P, the latter only can be supplied by Φ80. That λN mutants are efficiently helped by Φ80 does not tell that Φ80 provides the defective λ with an active N product; the λ chromosomes are simply packaged into a Φ80 coat. This shows that Φ80 is unable to switch on the late genes of λ. That neither Φ80 nor any of the Φ80hy tested can provide an active N product is shown in a more direct way by their complete failure to help λN -r14; this phage carries a polar mutation which makes the expression of genes O and P entirely N-dependant. The maturation of a λN - by Φ80 contrasts with the fact that mutants affected in late genes (A, F or H) are not efficiently helped by Φ80. This suggests that the products coded by these genes are not interchangeable between Φ80 and λ, and that packaging of λ DNA into Φ80 coats is possible but inhibited when λ late proteins are present in the cell. Activation of the Late Genes. Among the im ϕ80 h λ + hybrids tested, only Φ80hy41 is able to switch on the late genes of a λN defective mutant. This hybrid differs from the other hybrids studied here, by the fact that it has the Q-S-R region of λ (see Fig. 1). The results are consistant with the view that the product of Q gene is sufficient for activating the late genes of a λ DNA. N would thus control the expression of late genes only indirectly by controlling the expression of gene Q (Couturier & Dambly have independantly reached the same conclusion, 1970). Furthermore the failure of Φ80 and of the Φ80hy1 and Φ80hy4 to activate the late genes of λ would imply that these phages are unable to provide an Q product active on the λ chromosome Reciprocally, λ switches on the late genes of prophage Φ80hy41, but not of prophages Φ80hy1 and Φ80hy4. This suggests that the initiation of late genes expression takes place at a main specific site located in the Q-S-R region of the λ chromosome. The expression of the late genes would thus be sequential, and proceed through the left arm only when steaky ends cohere. Similar conclusions were reached independantly by Toussaint (1969) and by Herskowitz and Signer (1970).

Common Steps in the Repair of Alkylation and Radiation Damage in Yeast

Abstract: Radiation sensitive mutants of Saccharomyces cerevisiae were exposed to the action of nitrogen mustard (HN2) and methyl methanesulfonate (MMS). Sensitivity to HN2 was found to be correlated with sensitivity to ultraviolet light, whereas sensitivity to MMS was found to be correlated with sensitivity to X-rays. One mutant strain that is sensitive to both UV and X-rays was found to be sensitive also to HN2 and MMS. The latter result shows that there exists a locus in yeast that controls the repair of DNA damaged by all four of these mutagens.

Differential liquid holding recovery for the lethal effect and cytoplasmic petite induction by uv light in saccharomyces cerevisiae.

Abstract: Haploid and diploid wild types and UV-sensitive (uvs1–3) strains were exposed to UV light in stationnary phase of growth and in log phase. The liquid hold recovery (LHR) was studied in both conditions. 1. It appears that haploid wild type resting cells (1st type of repair) are less capable of repair during dark holding than dividing cells (2nd type of repair). 2. The mutant uvs1–3, which behaves like an excision defective strain, has lost the 1st type of repair. In contrast, the 2nd type of repair is still present. 3. The LHR is not additive to photorestoration (PR) for the 1st type of repair. On the contrary LHR and PR are additive for the 2nd type of repair. 4. Caffeine suppresses the 1st type of recovery and has only a slight effect on the 2nd type. 5. Both types of repair are functionning in diploid wild type cells where only the 2nd type of repair is present in UV-sensitive homozygous diploids uvs1/uvs1.

Four radiation sensitive mutants of Saccharomyces. Survival after UV- and x-ray-irradiation as well as UV-induced reversion rates from isoleucine-valine dependence to independence.

Abstract: Three not closely linked genes (r1s, r2sand r3s), a mutation in each of which causes UV- and X-ray-sensitivity of the colony forming ability in haploid and homozygous diploid cells have been identified. In one gene, r3s, two mutations have occurred independently. The effect of the mutated genes on long time repair processes as well as the effect on reversion rates from isoleucine-valine dependence to independence has been investigated. A model to explain the mechanisms changed by the mutated genes is discussed.

Molybdate metabolism in Aspergillus nidulans. II. Mutations affecting phosphatase activity or galactose utilization.

Abstract: Certain mutants ofAspergillus nidulans defective in galactose utilization are resistant to molybdate. However, mutational loss of galactokinase and/or galactose-1-phosphate uridyl transferase does not lead to molybdate resistance. Another class of molybdate resistant mutants utilizes galactose faster than the wild type and is hypersensitive to acriflavine. Certain mutations affecting phosphatase activity lead to resistance or hypersensitivity to molybdate. The pattern of molybdate tolerance among phosphatase mutants suggests that loss of an acid phosphatase (phosphatase IV) is associated with molybdate resistance while enhanced levels of this phosphatase produce hypersensitivity. It is concluded that a phosphorylated entity and a carbohydrate-containing compound synthesized from glucose-1-phosphate, uridine diphosphoglucose, or uridine diphosphogalactose participate in molybdate metabolism inA. nidulans and are possibly involved in molybdate uptake.

The effect of cycloheximide upon polyribosome stability in two yeast mutants defective respectively in the initiation of polypeptide chains and in messenger RNA synthesis

Abstract: The effect of cycloheximide upon protein synthesis, RNA metabolism, and polyribosome stability was investigated in the parent and in two temperature-sensitive mutant yeast strains defective respectively in the initiation of polypeptide chains and in messenger RNA synthesis. Cycloheximide at high concentrations (100 μg/ml) severely inhibits but does not completely stop protein synthesis (Fig. 1); the incorporation of 14C-amino acids into polyribosome-associated nascent polypeptide chains continues at a slow but measurable rate (Figs. 2 and 3). Polyribosome structures are stable in the parent strain at 36° whether or not cycloheximide is present (Fig. 5). However, in Mutant ts- 136, a mutant defective in messenger as well as in stable RNA production, polyribosomes decay at the restrictive temperature (36° C) at the same rate whether or not cycloheximide is present (Fig. 5). Thus the maintenance of polyribosome structures is dependent upon the continued synthesis of messenger RNA even under conditions of extremely slow polypeptide chain elongation. In mutant ts- 187, a mutant defective in the initiation of polypeptide chains, all of the polyribosomes decay to monoribosomes within 2 minutes after a shift to the restrictive temperature; cycloheximide completely prevents this decay demonstrating that this mutant is capable of continued messenger RNA synthesis at 36° C. Consistent with these observations is the fact that a newly synthesized heterogeneously sedimenting RNA fraction continues to enter polyribosomes in the presence of cycloheximide whereas the entrance of newly synthesized ribosomal RNA is severely inhibited (Figs. 7, 8, 9). The decay or lack of decay of polyribosomes at the restrictive temperature is, therefore, a rapid and discriminating test for the analysis of mutants defective in macromolecule synthesis. Mutants which exhibit a decay of polyribosomes in the presence of cycloheximide are likely to be defective directly or indirectly in the synthesis of messenger RNA whereas mutants in which decay is prevented or slowed by cycloheximide are likely to be defective in some factor required for the association of ribosomes and messenger RNA.

Ultraviolet-sensitive mutants of Neurospora. II. Radiation studies.

Abstract: Two new, recessive, chromosomal mutants of Neurospora, uvs-3, and uvs-4, have been found to be more sensitive to UV than wild type. The uvs-3 mutant, is very sensitive to nitrogen mustard, nitrosoguanidine and X-rays. UV-dose-response curves of uvs-3 lack the shoulder typical of wild type but parallel the wild-type curve at low survival levels. Unlike wild type, which shows a decrease in survival when incubated in water or proflavine (4 mg/ml) after UV, uvs-3 shows an increase in survival after water incubation and an even greater increase when incubated in proflavine. It also differs from wild type in showing much less photoreactivation and in not losing its photoreactivation ability during two hours incubation in water. The uvs-4 mutant is not sensitive to nitrogen mustard and only slightly sensitive to nitrosoguanidine. Dose-response curves of uvs-4 retain a shoulder and are steeper than those of wild type at low survival levels. Proflavine sensitivity and photoreactivation of uvs-4 resemble wild type, but no decrease in survival occurs when it is held in water after UV. The combination of characteristics shon, by uvs-4 is not like that of any of the UV-sensitive mutants found in E. coli but those of uvs-3 resemble those shown by the EXR (X-ray sensitive, low to normal recombination) and reckless REC (X-ray sensitive, recombinationless) mutant classes in bacteria.

Mutants of Escherichia coli K12 (λ)+ non-inducible by thymine deprivation

Abstract: A selective enrichment method based upon differential killing by thymine deprivation of inducible as compared to non-inducible lysogens was employed to isolate mutants of Escherichia coli K 12(λ)+ deficient in lysogenic induction. The efficiency of the method is such that about 1% of the surviving colonies are resistant to thymineless induction.

Pleiotropic effects of de-repressed bacterial sex factors on colicinogeny and cell wall structure.

Abstract: Colonies of Escherichia coli or Salmonella typhimurium that form colicin I often produce larger inhibition zones when the sex factor of their plasmid is de-repressed. In liquid culture, virtually all colicin I is cell-bound; colicin titres with de-repressed factors are only slightly greater than with the wild type; no more covalently-closed plasmid DNA is present; and no more plasmid-determined enzyme is formed. The large zones are therefore unlikely to reflect an increase in the number of plasmid genomes per cell. De-repressed factors make the cells susceptible to lytic agents, indicating a change in the cell wall, which may result in greater release of cell-bound colicin and so increase the size of inhibition zones. Salmonella typhimurium LT2 carries a plasmid of unknown function.