Female-specific phages and F-minus strains of Escherichia coli K12

Summary The female-specific phages, I, II, W31 and H (but not T3 and T7) show a low efficiency of plating on all F-minus strains of Escherichia coli K12 except for the thr leu thi mutants descended from strain Y53. The locus responsible is linked to the histidine region and was presumably eliminated from the Y53 line of mutants in the course of mutagenesis.     

Effect of ruv mutations on recombination and DNA repair in Escherichia coli K12

Summary Mutation of the ruv gene of E. coli is associated with sensitivity to radiation, and filamentous growth after transient inhibition of DNA synthesis. The filamentation of ruv strains is abolished by mutations in sfiA or sfiB that prevent SOS induced inhibition of cell division, but this does not restore resistance to UV radiation. Double mutants carrying both ruv and uvr mutations are considerably more sensitive to UV radiation than the single mutants, but there is no additive effect of ruv with recA, recF, recB, or recC mutations. ruv mutations have little effect on conjugal recombination in wild-type strains but confer recombination deficiency and extreme sensitivity to ionizing radiation in recBC sbcB strain. These results, together with the fact that ruv strains are excision proficient and mutable by UV light, are interpreted to suggest that the ruv ⁺ product is involved in recombinational repair of damaged DNA rather than in cell division as suggested by Otsuji et al. (1974).     

Repair of DNA double-strand breaks in Escherichia coli K12 requires a functional recN product

Summary Mutation of the recN gene of Escherichia coli in a recBC sbcB genetic background blocks conjugational recombination and confers increased sensitivity to UV light and mitomycin C. The basis for this phenotype was investigated by monitoring the properties associated with recN mutations in otherwise wild-type strains. It was established that recN single mutants are almost fully resistant to UV irradiation, and that there is no detectable defect in repair of UV lesions by excision, error-prone, or recombinational mechanisms. However, recN mutations confer sensitivity to mitomycin C and ionizing radiation both in wild-type and recB sbcB strains. The sensitivity to ionizing radiation is correlated with a deficiency in the capacity to repair DNA double-strand breaks by a UV inducible mechanism. Recombinant phages that complement the recombination and repair defects of recN recBC sbcB mutants have been identified, and the recN gene has been cloned from these phages into a low copy-number plasmid.     

Molecular cloning of the fnr gene of Escherichia coli K12

Summary Mutations in the fnr gene of Escherichia coli have pleiotropic effects leading to deficiencies in the reduction of fumarate and nitrate, hydrogen production and the ability to grow anaerobically with fumarate or nitrate as terminal electron acceptors. Transducing phages (fnr) carrying the wild-type fnr gene were isolated from populations of artificially-constructed recombinant lambda phages by their ability to complement the lesions of fnr mutants. The fnr phages restored anaerobic growth with fumarate and nitrate as electron acceptors and, as prophages, they promoted normal synthesis of fumarate reductase, nitrate reductase and hydrogenase in fnr mutants. Five independently-isolated fnr phages each contained a R.HindIII fragment (11.5 kilobases) that possessed three internal R.EcoRI targets and had inserted with the same orientation relative to the phage. A physical map of the fnr region was constructed by restriction analysis and flanking fragments were identified by DNA:DNA hybridization.     

High efficiency temperature-sensitive amber suppressor strains of Escherichia coli K12

Summary A set of eight strains combining the supD43,74 ts¹ suppressor gene with alleles of three suppressor-enhancing (sue) genes have been constructed and characterized. The sue mutations work cooperatively to raise suppressor activity and together raise the activity of the supD43, 74-encoded suppressor 40-fold. These strains further expand the utility of the ts suppressor system by providing as much as 100% suppressor activity at temperatures at or below 20°C to as little as 0.015% suppressor activity at 43°C.     

A temperature-sensitive mutant in prolinyl-tRNA ligase of Escherichia coli K-12

Summary A mutant with a defective prolinyl-tRNA ligase has been found in a collection of spontaneous temperature-sensitie mutants. The mutated gene, which is designated proS, is closely linked to metD.     

Effect of ogt expression on mutation induction by methyl-, ethyl- and propylmethanesulphonate in Escherichia coli K12 strains

We have previously reported the isolation of an Escherichia coli K12 mutant that is extremely sensitive to mutagenesis by low doses of ethylating agents. We now show by Southern analysis that the mutation involves a gross deletion covering at least the ogt and fnr genes and that no O⁶-alkylguanine-DNA-alkyltransferase activity is present in cell-free extracts of an ada::Tn10 derivative of these bacteria. Confirmation that sensitisation to ethylation-induced mutagenesis was attributable to ogt and not to any other loci covered by the deletion was obtained by constructing derivatives. Thus an ogt::kan^r disruption mutation was introduced into the parental ogt ⁺ bacteria, and the ogt::kan^r mutation was then eliminated by cotransduction of ogt ⁺ with the closely linked Tet ^r marker (zcj::Tn10). The (ogt-fnr) deletion or ogt::kan^r disruption mutants were highly sensitive to ethyl methanesulphonate-induced mutagenesis, as measured by the induction of forward mutations to l-arabinose resistance (Ara¹). Furthermore, the number of Ara^r mutants increased linearly with dose, unlike the case in ogt ⁺ bacteria, which had a threshold dose below which no mutants accumulated. Differences in mutability were even greater with propyl methanesulphonate. Overproduction of the ogt alkyltransferase from a multicopy plasmid reduced ethylmethanesulphonate-induced mutagenesis in the ogt mutant strains and also methylmethanesulphonate mutagenesis in ada ^– bacteria. A sample of AB 1157 obtained from the E. coli K12 genetic stock centre also had a deletion covering the ogt and fnr genes. Since such deletions greatly influence the mutagenic responses to alkylating agents, a survey of the presence of the ogt gene in the E. coli K12 strain being used is advisable.     

Biosynthesis of single-carbon units in Escherichia coli K12

Two routes of single-carbon (C₁) biosynthesis have been described in Escherichia coli K12 previously: from serine via serine transhydroxymethylase and from glycine via a little known set of enzymes, the Glycine Cleavage Pathway. Strains deficient in both of these still have a source of C₁. There therefore exists a third pathway of C₁ formation. Serine appears to be the donor for the pathway. No donor of C₁ other than serine of glycine appears to be of quantitative importance in E. coli K12 growing in a synthetic glucose-supplemented medium.     

On the mechanism of conjugation in Escherichia coli K 12

Summary The phenomenon of conjugation consists of many stages. The most important are: the formation of contacts between mating cells, the transfer of DNA from the donor to the recipient, and the integration of the transfered DNA fragments into the chromosome of the recipient. Only after completion of all these stages are recombinants formed. With the aid of specific inhibitiors (nalidixic acid, FUDR), thymine starvation, and use of special thermosensitive mutants it is possible to study the role of DNA synthesis during every stage of conjugation. It was demonstrated that the genetic transfer is due to semiconservative DNA-replication in the donor cell. The fragments of DNA transfered are synthesized in the period of mating by a special replication system (F-replicon). In case of T _DNA ^S mutants unable to grow at 41°, the ability to synthesize DNA during conjugation is preserved.The inhibition of the DNA synthesis in the donor cell by poisons leads to complete inhibition of genetic transfer. The third stage — formation of recombinants requires DNA synthesis in the recipient cell and is inhibited by poisoning, thymine starvation or T _DNA ^S mutations in the recipient. In cases where recombination is not involved (i.e. sexduction) the inhibition of DNA synthesis in the recipient has no significant effect.     

Isolation of plasmids carrying the arginine repressor gene argR of Escherichia coli K12

Summary Twenty-two sexual crosses between strains of Phycomyces blakesleeanus carrying mutations affecting phototropism (madA, madD, madE), synthesis of carotenoids (carA), auxotrophy (leu-51, nicA, pur-51), and resistance to 5-fluorouracil (fur) were studied; mating type was also included as a marker. Recombination frequencies were obtained among the ten genes involved. Linkage was found between mating type and madE; leu-51 and madA; furA401, furB402 and madD. All other gene combinations tested are unliked.     

DNA degradation in minicells of Escherichia coli K-12

Summary The properties of minicell producing mutants of Escherichia coli deficient in gentic recombination were examined. Experiments were designed to test recombinant formation in conjugal crosses, survival following UV-irradiation in cells, and the state of DNA metabolism in minicells. The REC^- phenotypes are unaffected by min ⁺/^- genotypes in whole cells. In contrast to minicells produced by rec ⁺ parental cells, minicells from a recB21 strain have limited capacity to degrade linear, Hfr transferred DNA. The lack of a functional recA gene product, presumably involved in inhibiting the recBC nuclease action(s), permits unrestricted Hfr DNA breakdown in minicells produced by a recA1 strain. This results in an increase in TCA soluble products and in the formation of small DNA molecules that sediment near the top of an alkaline sucrose gradient. Unlike the linear DNA, circular duplex DNA from plasmids R64-11 or dv, segregated into the minicells, is resistant to breakdown. By using in vitro criteria, and [³²P]-labelled linear DNA from bacteriophage T₇ for substrate, we found that the ATP-dependent exonuclease of the recBC complex (exo V) is present in rec ⁺ and recA^- minicells, and is lacking in the recB21 mutant. In fact, the absence of a functional exo V in recBC^- minicells results in isolation of larger than average Hfr DNA from minicells. We suggest that recombination (REC) enzymes segregate into the polar minicells at the time of minicell biogenesis. This system should be useful for studies on DNA metabolism and functions of the recBC and recA gene products.Paper 1 in series, see Khachatourians et al., 1974.     

Temperature dependent survival of UV-irradiated Escherichia coli K12

Summary We have found that several excision deficient derivatives of Escherichia coli K12 survive better after UV irradiation if incubated at 42°C than if incubated at 30°C. The highest survival was observed when incubation at 42°C followed UV irradiation and was maintained for at least 16 h. Our results indicate that this temperature dependent resistance (TDR) requires a functional recA gene, but not uvr A, uvrB, recF, or recB genes, or the recA441 (tif-1) mutation which allows thermoinduction of the recA-lexA regulon. Our data are consistent with the idea that the increase in survival observed at 42°C reflects enhanced daughterstrand gap repair by DNA strand exchange. Although the conditions used to elicit TDR can induce heat shock proteins and thermotolerance in E. coli, the relationship between the two responses remains to be elucidated.     

Curing of Escherichia coli K12 plasmids by coumermycin

Summary Low concentrations of the antibiotic coumermycin A₁, the inhibitor of bacterial DNA gyrase, effectively eliminate pBR322, pMB9 and other ColE1 related plasmids from E. coli K12 strains. The curing action of antibiotic seems to result from the plasmid degradation and not just from the inhibition of replication.     

Regulatory mutants of dihydrofolate reductase in Escherichia coli K12

Summary Trimethoprim inhibits dihydrofolate reductase. Mutations conferring trimethoprim-resistance on E. coli K12 result in either an altered reductase with decreased affinity for the drug, or in 2–30 fold higher levels of the enzyme. Studies of the latter class of mutants indicate that dihydrofolate reductase is regulated by a diffusible molecule, and is probably under negative control. The regulatory mutants, some of which are temperature-sensitive, act cis.     

Deletion map of the Escherichia coli K-12 dnaB gene

Summary Acid phosphatase isoenzymes of Chlamydomonas reinhardii were investigated by isoelectric focusing in polyacrylamide gel systems. In this paper we describe in detail an original method for isoelectric focusing of acid phosphatases extracted from wildtype and acid phosphatase-lacking mutant algae, obtained from Laboratoire de Génetique of University of Liège. Three isoenzymes can be separated from the buffer-soluble components of these cells. An additional isoenzyme type can be visualized using the nonionic detergent NP40 as solubilizer. We conclude that these four isoenzymes are releated to the structural gene of the soluble constitutive acid phosphatase, which was shown by their appearance in P ₂ and their total absence in mutant P _a. The pl values of soluble constitutive acid phosphatase isoenzymes range between pH 5.2 and 6.2. As a result of treatment with NP40 the extracts from both wild-type and mutant lines contain two additional active phosphatase forms which can be characterized by their high heat resistance and low pI values. These enzymes are fully active using either -naphthyl phosphate or different acetate esters as substrates.