Low recombination frequency for markers very near the origin in conjugation in E. coli

Studies on bacterial conjugational crosses via model populations. III. Reproduction of results of Hfr x F- crosses in Escherichia coli

Hfr x F- recombinational genome interactions are simulated by a computer program. Genotype spectrums of selected samples from Hfr x F- crosses of Escherichia coli, found in the published literature, are reproduced. Relevance of the findings for interpretation of conjugational results in Escherichia coli is discussed.

Spontaneous zygogenesis in Escherichia coli, a form of true sexuality in prokaryotes

A new type of mating, differing from classic conjugation and previously observed in a certain strain of Escherichia coli K-12, has also been found in strains derived from ordinary F- cells of E. coli K-12 exposed to an exogenous factor originating in an E. coli clinical isolate. Immunofluorescence and electron microscopy after single and double labelling of DNA were used to produce evidence in favour of a novel mating mechanism by cell contact at the poles of the bacterial rod. These findings are supported by genetic analyses indicating complete genetic mixing. Unstable complementing diploids were formed, which throw off phenotypically haploid cells, of which some showed a parental phenotype and some were true genetic recombinants. Recombination was observed even when one parent was a UV-inactivated F- RecA- strain. The name 'spontaneous zygogenesis' (Z-mating, for short) is proposed for this kind of mating.

Conjugational hyperrecombination achieved by derepressing the LexA regulon, altering the properties of RecA protein and inactivating mismatch repair in Escherichia coli K-12

The frequency of recombinational exchanges (FRE) that disrupt co-inheritance of transferred donor markers in Escherichia coli Hfr by F(-) crosses differs by up to a factor of two depending on physiological factors and culture conditions. Under standard conditions we found FRE to be 5.01 +/- 0.43 exchanges per 100-min units of DNA length for wild-type strains of the AB1157 line. Using these conditions we showed a cumulative effect of various mutations on FRE. Constitutive SOS expression by lexA gene inactivation (lexA71::Tn5) and recA gene mutation (recA730) showed, respectively, approximately 4- and 7-fold increases of FRE. The double lexA71 recA730 combination gave an approximately 17-fold increase in FRE. Addition of mutS215::Tn10, inactivating the mismatch repair system, to the double lexA recA mutant increased FRE to approximately 26-fold above wild-type FRE. Finally, we showed that another recA mutation produced as much SOS expression as recA730 but increased FRE only 3-fold. We conclude that three factors contribute to normally low FRE under standard conditions: repression of the LexA regulon, the properties of wild-type RecA protein, and a functioning MutSHL mismatch repair system. We discuss mechanisms by which the lexA, recA, and mutS mutations may elevate FRE cumulatively to obtain hyperrecombination.

Ufr/s variation in Escherichia coli K12: a reversible double-mutation or alternate chromosome expression in non-complementing diploids?

A novel form of bacterial variation found in an FhuA- mutant of Escherichia coli K12 was characterized by the alternation of (1) simultaneous resistance to lipopolysaccharide-specific phage U3 and to FhuA-specific agents (Ufr phenotype); and (2) a return to the sensitivity pattern of the initial strain (Ufs). In Ufr cells, loss of the U3 receptor permitted C21 adsorption without modifying the sensitivity to other tested phages or colicins. Genetic analysis revealed that Ufr variants were altered at two distinct loci. Ufr bacteria, though derived from a strain F- devoid of classical gene transfer mechanisms, were transiently able to promote mating between themselves and, to some extent, with other bacteria, including Rec-. Heterogenic matings resulted in the formation of persistent heterozygotes segregating Ufr- and Ufs-like bacteria. Pedigree analysis and subcloning of heterozygotic isolates indicate that they were diploids, as was the initial Ufr strain. Functional genetic complementation between these two genomes was only transient and the alternative forms were likely to result from the expression of a single chromosome of the heterozygotes. Mutation occurred in either form without causing any change in the alternative form.

Functional structures of the recA protein found by chimera analysis

We developed a novel genetic method for finding functional regions of a protein by the analysis of chimeras formed between homologous proteins. Sets of chimeric genes were made by intramolecular homologous recombination in a linearized plasmid DNA carrying both recA genes of Escherichia coli and Pseudomonas aeruginosa. A recBCsbcA strain of E. coli was used for isolation of plasmids carrying recombinants between these genes. Examination of properties of E. coli strains deleting the recA gene and carrying a plasmid with a chimeric gene shows that chimera formation at certain positions inactivates a RecA function. Frequently, all chimeras with a junction in a certain region of the protein inactivate a function. Rather than a direct effect of the presence of the junction at a particular position, mismatching of the regions both sides of the junction that are derived from the different species is responsible for the inactivation. For a chimeric protein to be functional, certain pairs of sequences in different regions of the protein must derive from the same parent. Four pairs of such sequences were found: two are involved in activities for genetic recombination and for resistance to ultraviolet light irradiation and the others in formation of active oligomers. Regions defined by these sequences are located in the looped regions of the protein. A pair of regions may co-operate to form a functional folded structure.

The use of lambda plac-Mu hybrid phages in Klebsiella pneumoniae and the isolation of stable Hfr strains

Klebsiella pneumoniae 1033-5P14 and its P1-sensitive derivative KAY2026 were found to be resistant to lambda although they contained a LamB protein, active as a maltoporin. Sensitive derivatives could only be obtained after introduction of the pTROY9 plasmid which expresses lamB and the corresponding lambda receptor from Escherichia coli K12 at high levels. Lysogenic derivatives from such strains were shown to carry the phage at secondary att sites and to give high titer lysates when induced. The use of lambda plac-Mu hybrid phages allowed the isolation from several operons of lacZ fusions orientated in, or against, the direction of transcription. Such insertions could subsequently be used to isolate stable Hfr strains by allowing homologous recombination to take place between the lac genes in the inserted hybrid phages and those of plasmid F' ts114 lac+ zzf20::Tn10. The Hfr strains were able to transfer K. pneumoniae chromosomal genes and allowed the mapping of such genes. Characteristic differences between this conjugation system and that of Escherichia coli K12 are discussed. The insertions also allowed determination of the direction of transcription of the gut gene, the newly mapped scr gene and of the sor gene cluster encoding enzymes for the metabolism of D-glucitol, sucrose and L-sorbose.

Characterization of a mutation conferring radiation sensitivity, ior, located close to the gene coding for deoxycytidine deaminase in Escherichia coli

The isolation and characterization of a new mutation conferring radiation sensitivity in Escherichia coli is described. This mutation is located close to the gene coding for deoxycytidine deaminase, in the chromosomal region of the gat operon. It is very sensitive to gamma rays and exhibits a decrease in recombination ability. The expression of radiation sensitivity seems to result from the additive effect of the dcd mutation and another mutation of unknown function.

Conjugal transfer of bacterial chromosomes mediated by the RK2 plasmid transfer origin cloned into transposon Tn5

We report here a novel system for the conjugal transfer of bacterial chromosomes which utilizes the transfer origin (oriT) of plasmid RK2 cloned into transposon Tn5. Tn5 with oriT was inserted by transposition into the chromosomes of Escherichia coli and Rhizobium meliloti. The oriT sequence then served as the origin of high-frequency chromosome transfer when a helper RK2 plasmid was present in the same cell. The broad host range features of RK2 make this system of oriented chromosome mobilization applicable to most gram-negative bacteria.

Site-specific recombination following conjugation in Escherichia coli K-12

In accord with the observations of other workers, unselected marker analysis of Escherichia coli K-12 transconjugants isolated from matings involving several different Hfr strains as donors has shown that most genetic exchanges are clustered either near the selected marker or near the origin of the transferred Hfr DNA. The present work increases the number of Hfr strains tested and shows that the clustering of the recombinational events near the origin of transfer is statistically significant. It is proposed that this clustering of genetic exchanges is due to the action of a unique recombination system (site-specific conjugal recombination or ssr) which recognizes the early transferred portion of the F plasmid and catalyzes a genetic exchange in or near the adjacent bacterial DNA. Twelve Hfr strains representing eleven different points of origin were tested, and only KL16 and Ra-1 did not demonstrate the typical clustering of genetic exchanges. Since these strains share a common transfer origin, they may represent spontaneous mutations affecting the ssr system.

Convenient construction of recA deletion derivatives of Escherichia coli

Two Escherichia coli K-12 Hfr strains have been constructed which transfer a recA deletion, which is highly linked to a Tn10 insertion conferring tetracycline resistance, early during conjugation. These strains transfer the recA deletion in opposite directions with different origins of transfer, allowing for preservation of desirable recipient strain markers either clockwise or counterclockwise of recA.

Use of a known plasmid and transposon as tracers for the incompatibility classification of a cryptic plasmid

We have developed a novel genetic technique by which an unknown plasmid can be classified by the use of plasmid of known incompatibility group. In phenocopy state, cells harboring plasmids of known incompatibility group behave as normal recipients and receive plasmids belonging to the same incompatibility group at a high frequency. This work provides additional evidence that plasmids in 'phenocopy' hosts do not replicate and therefore fail to demonstrate their incompatibility barrier to the incoming plasmids. A cryptic plasmid, now called pWS7, has been identified by this method in a pili, fla female strain of E. coli K12. Genetic analysis shows the plasmid pWS7 is in fact, a sex-factor which is curable with acridine orange. It belongs to the Inc F1 group. Physical analysis confirms its size to be 124 Kb. The plasmid has been labelled genetically with a transposon Tn903 in a recA host and further characterized by heteroduplex analysis. A DNA sequence homology between pWS7 and F'lac plasmid extends only in F-regions, 2.8F-94.5F. The pili, fla host strain of pWS7 shows a high frequency of transformation for recombinant DNA and rapid propagation for a male-specific RNA phage, R17.

Mapping of two transcription mutations (tlnI and tlnII) conferring thiolutin resistance, adjacent to dnaZ and rho in Escherichia coli

Two mutations in Escherichia coli conferring resistance to the transcription initiation inhibitor, thiolutin, have been mapped. One of these mutations (tln-I)( maps at 10.2 min on the genetic map and is cotransducible with dnaZ at a frequency of approximately 50%. The other mutation (tln-II) maps between metE and ilvD, probably close to rho, and is cotransducible with ilvD at a frequency of approximately 65%. The presence of both the mutations in the same cell confers resistance to thiolutin in minimal medium. Either one of them alone renders the cell 'conditionally auxotrophic' in the presence of the drug. The implications of these findings are discussed in relation to the mode of action of the thiolutin sensitive factors in transcription.

Isolation and identification of mutants constitutive for aspartokinase III synthesis in Escherichia coli K 12

We devised a procedure in order to isolate, in Escherichia coli, constitutive mutants for aspartokinase III synthesis, the first enzyme of the lysine regulon. It consists of the introduction of a limiting step in lysine biosynthesis, by the use of the partial suppression of a nonsense mutation. For the first time we could isolate many constitutive mutants. Their characteristics (cotransduction with the lysC structural gene; no effect on the synthesis of other enzymes of the regulon; cis-dominance) lead to classify these mutations as operator-type. The fact that no repressor mutations could be isolated is discussed.

Inversion in the lactose region of Escherichia coli K-12

A spontaneous mutant of Escherichia coli K-12, strain SY99, with an inversion in the lactose region was isolated and partially characterized. The inversion was detected due to inverse chromosomal conjugational transfer after introduction of an F42 (F'lac) episome. The termini of the inversion are between proAB and lac on one side and lac and proC on the other. The inverse conjugational transfer in SY99 did not appear to be absolute but was always accompanied by a residual "normal" counterclockwise mobilization. This residual transfer was further shown to be caused by the intrinsic instability of this region (at least in the line W3110). The possible involvement of IS3 elements flanking the lactose operon is discussed.

Genetic analysis of Escherichia coli O111:B4, a strain of medical and biochemical interest

Procedures have been worked out which allow, for the first time, the genetic analysis of Escherichia coli O111:K58:H2 (O111:B4). The approximate map position of mutant loci was determined by mating with 15 Hfr strains of E. coli K-12. In addition, P1 transduction procedures were used for establishing relative gene order and linkage for any region of the E. coli O111:B4 chromosome. To obtain these, it was necessary to select for a rare P1 lysogen since E. coli O111:B4 is resistant to phage P1. Finally, genetic homology between E. coli strains K-12 and O111:B4 is suggested since they can form stable haploid hybrids, and several loci have similar map positions in the two strains.

Stringent control of peptidoglycan biosynthesis in Escherichia coli K-12

[3H]Diaminopimelic acid (Dap) was incorporated exclusively into peptidoglycan by Escherichia coli strains auxotrophic for both lysine and Dap. The rate of [3H]Dap incorporation by stringent (rel+) strains was significantly decreased when cells were deprived of required amino acids. The addition of chloramphenicol to amino acid-starved rel+ cultured stimulated both peptidoglycan and ribonucleic acid synthesis. In contrast, a relaxed (relA) derivative incorporated [3H]Dap at comparable rates in the presence or absence of required amino acids. Physiologically significant concentrations of guanosine 5'-diphosphate 3'-diphosphate (ppGpp) inhibited the in vitro synthesis of both carrier lipid-linked intermediate and peptidoglycan catalyzed by a particulate enzyme system. The degree of inhibition was dependent on the concentration of ppGpp in the reaction mixture. Thus, the results of in vivo and in vitro studies indicate that peptidoglycan synthesis is stringently controlled in E. coli.

Fusion of two F-prime factors in Escherichia coli studied by electron microscope heteroduplex analysis

A fused F prime factor was obtained from a mating of a recA donor carrying an F'- factor containing the genes metBJF, ppc and argECBH (KLF5) with a recA recipient carrying an F' factor containing att80, trp and lac (f155). lysogenization of this fused F-prime factor with gammacI857hphi80 phage followed by thermoinduction produced the transducing phages phi80 dmetBJF and phi80 dppcargECBH. This kind of fusion provides a general procedure for the construction of transducing phages carrying genes from different regions of the E. coli genome. To understand the mechanism of this fusion, the parental F prime factors (F155 and KLF5) were analyzed by the electron microscope heteroduplex technique. F155 has a length of 176 +/- 3 kilobases including two substitutions. The F sequence 0 F-2.8 F has been substituted by 53 kb of chromosomal DNA including the lac operon and the F sequences 8.5 F-16.3 F has been substituted by 27 kb of a chromosome sequence including att80 and the trp operon KLF5 contains 221 +/- 4 kilobases of DNA (molecular weight, 148 megadaltons). It contains complete F and the segment of the E. coli chromosome from polA to rif. The F sequence 2.8 F-8.5 F known to be involved in F specific recombination in recA+ and recA backgrounds occurs twice on KLF5, once at each of the junctions of F DNA with chromosomal DNA. The population of closed circular plasmid molecules extracted from KLF5-containing strains is heterogeneous. It is proposed that this heterogeneity is due to intramolecular recombination events occurring in KLF5 between the duplicated 2.8 F-8.5 F sequences. Such recombination can account for the genetic instability of KLF5 observed in both recA+ and recA hosts. The F sequence 2.8 F-8.5 (also called gammadelta) is one of the characterized integration sequences on F. A model for the fusion of the parental F prime factors is proposed in which recombination between gammadelta sequences brings att80 close to the metBJF genes. This is followed by a deletion of an F' lac factor. The resulting fused F' factor still carries two gammadelta sequences and is therefore expected to be unstable. The closed circular molecules isolated from the fused F' containing strains show two different sizes of molecules. Genetic and physical analyses of these molecules are in agreement with the predicted instability of the fused F' factor and the existance of the gammadelta sequence in the phi80 dmet phages isolated from fused F' and previously analyzed by the electron microscope heteroduplex technique.

Mutants of Salmonella typhimurium deficient in an endoprotease

Three bands of hydrolytic activity toward the chromogenic protease substrate N-acetyl-DL-phenylalanine beta-naphthyl ester (NAPNE) can be observed after gel electrophoresis of crude extracts of Salmonella typhimurium or Escherichia coli. Mutants deficient in one of these three activities have been isolated using a staining procedure that identifies colonies that show reduced ability to hydrolyze NAPNE. These mutants lack the strongest of the three bands of activity. The Salmonella mutations (designated apeA) are all co-transducible with purE, and the order (pro)-apeA-Hfr K17 origin-purE has been established. Strains carrying apeA mutations have wild-type doubling times. None of the apeA mutants isolated gains an auxotrophic requirement as a result of loss of the apeA gene product. The rates and extents of protein degradation during starvation for a carbon source or during growth after exposure to the amino acid analogue canavanine do not seem to be affected by apeA mutations. Revertants of apeA mutations (selected by screening for clones that have regained the ability to hydrolyze NAPNE) frequently contain a new enzymatic activity not found in wild-type cells.

Genetic mapping of xthA, the structural gene for exonuclease III in Escherichia coli K-12

The genes xthA, pncA, and pabB were ordered relative to others by two- and three-factor transductional crosses with bacteriophage P1. The genes studied span 2 min (2%) of the genetic map of Escherichia coli K-12 in the clockwise sequence pheS-pfkB-xthA-pncA-gap-pabB-fadD. Eleven independently derived xth mutations were examined; all were known to affect exonuclease III and its associated endonuclease II activity, and all were mapped in the xthA region. pncA mutations were found to confer resistance to 6-aminonicotinamide, whereas some pheS mutations are known to specify resistance to p-fluorphenylalanine. xth mutations were readily transferred into other strains by selecting for these co-transducible drug resistance markers.

Mutants of Escherichia coli K12 unable to grow on non-fermentable carbon substrates

Among a number of mutants unable to utilize non-fermentable carbon substrates, scoring for membrane ATPase and for ATP-driven transhydrogenase activity permitted to distinguish two phenotypes: (A) mutants lacking ATPase and ATP-driven transhydrogenase; (B) one mutant with an ATPase which behaved according to several criteria as released into solution instead of being membrane bound, a.o it exhibited no ATP-driven transhydrogenase activity. All A and B mutants exhibited a common nutritional pattern. The ATPase-deficient group, when scored for ATPase-binding sites on its membrane particles revealed three different subgroups: (1) mutants having free ATPase-binding sites, (2) mutants with ATPase-binding sites made available by the procedure which releases ATPase from wild-type membrane, and (3) mutants with no detectable ATPase-binding sites. Membranes of the mutant B with unbound ATPase also exhibited a deficiency in ATPase-binding sites, but its soluble ATPase was also found unable to bind to ATPase-binding sites of wild type membranes. The double alteration, namely abnormal or inactive ATPase and absence of ATPase-binding sites on the membrane is compatible with a single mutational defect.

Mapping of the locus for Escherichia coli transfer ribonucleic acid nucleotidyltransferase

cca Mutants of Escherichia coli K12 have altered levels of tRNA nucleotidyl-transferase activity. The cca locus has been located at minute 59.4 of the E. coli linkage map. It is cotransduced with tolC but not with argG, and is the earliest known marker transferred by Hfr strain KL14. The proximity of the tolC locus to the integrated sex factor in Hfr strain KL14 may be useful for mapping sex factor mutations by transduction.

Conditional mutator gene in Escherichia coli: isolation, mapping, and effector studies

A mutator gene, mutD5, whose phenotype is conditional, has been identified in Escherichia coli. By P1 transduction it has been shown to lie at about 5.7 min on the chromosome, being co-transduced with proA and argF. In rich medium, streptomycin- and nalidixic acid-resistant mutation frequencies are 50 to 100 times higher than those in minimal medium. In minimal medium, the mutD5-induced mutation frequencies are still 50 to 100 times above co-isogenic wild-type (mut(+)) levels. Similar results were obtained with all markers tested. Mutant frequencies can be raised by thymidine in the medium at concentrations as low as 0.04 muM, or by the endogenous generation of thymidine from thymine plus a deoxyribosyl donor. Deoxyadenosine, various ribonucleosides, thymine, and 2-deoxyribose do not stimulate mutation. None of these effects are related to growth rate, since growth rate and mutation rate can be decoupled completely.

Rapid mapping of conditional and auxotrophic mutations in Escherichia coli K-12

The approximate genetic map locations of auxotrophic and conditional lethal mutations of Escherichia coli can be rapidly determined with replica plating techniques. A set of patches of 15 streptomycin-sensitive (Str(S)) Hfr strains with points of origin distributed around the map is replica plated onto a recombinant-selective plate with a lawn of Str(R) cells which carry an unmapped mutation. The map interval defined by the Hfr points of origin which are closest to the mutant locus is seen by the presence or absence of heavy patches of recombinants produced by transfer of early wild-type genes from the Hfrs. An alternative method is to replicate patches of different mutant strains (100 per plate) onto Hfr lawns; in this case more than 1,000 different mutants can be mapped in a single experiment in a few days. In this way, many types of mutations with similar phenotypes can be grouped as to approximate location on the genetic map. For ordering mutations within groups, the same replica plating methods can be used to cross F-prime derivatives of mutants with other mutants of the same group. Relative merits of these and other mapping methods of E. coli are discussed.

Mapping of the fabA locus for unsaturated fatty acid biosynthesis in Escherichia coli

fabA mutants of Escherichia coli require an appropriate unsaturated fatty acid for growth. The fabA locus has now been mapped at minute 21.5 of the linkage map of E. coli. The locus is cotransduced with pyrD and aroA but not with pyrC, purB, or pdxC. The clockwise order of markers in the region is pdxC, aroA, cmlB, pyrD, fabA, pyrC.

Insertion of the F factor into the cluster of rfa (rough A) genes of Salmonella typhimurium

From F(+) strains of Salmonella typhimurium, isolates were obtained representing two new classes of Hfr strains, HfrK1 and HfrK2, in which the insertion of the F factor into the rfa genes results in chromosome mobilization either clockwise or anticlockwise from rfa, and in the Rfa phenotype. The point of insertion of the F factor into the cluster of rfa genes, revealed by studies of the early transfer of their normal alleles, is as follows: xyl-cysE-rfa-657 (HfrK2-1, SA540 -->)-(<-- HfrK1-1, SA458)-rfaG-(<-- HfrK1-2, SA464)-pyrE-metA

Effect of Zn2+ on bacterial conjugation: inhibition of mating pair formation

Zn(2+) at 10(-3)m has been found to block the formation of mating pairs between Hfr and F(-) strains of Escherichia coli as observed both by light microscopy and by Coulter counter measurements. Kinetic studies show that Zn(2+) reduces the fertility of the male and that its effect disappears within 2 min after Zn(2+) has been removed from the medium. Short treatments of female cells with Zn(2+) have no detectable effect on their ability to form mating pairs. Later steps in the mating process such as mobilization of the male chromosome, transfer of the chromosome to the female, or its integration into the female chromosome seem not to be affected by 10(-3)m Zn(2+).

Two mutations giving low-level streptomycin resistance in Escherichia coli K 12

A mutation inEscherichia coliK12 giving resistance to about 5 μg/ml of streptomycin was found to be cotransducible by P1 withproAandproB, and is located at about 8·5 min on the chromosome map. The locus is namedstrB. A second mutation to the same resistance level was not cotransducible with eitherproAorproBand must be located elsewhere. Both mutations cause a marked increase in R-factor mediated streptomycin resistance, and significant decreases in resistance to several other antibiotics, both in the presence and absence of an R-factor determinant for the same antibiotic. The two mutations differ in their effects on bacterial sensitivity to crystal violet and EDTA.

Formation of aromatic amino acid pools in Escherichia coli K-12

Phenylalanine, tyrosine, and tryptophan were taken up into cells of Escherichia coli K-12 by a general aromatic transport system. Apparent Michaelis constants for the three amino acids were 4.7 x 10(-7), 5.7 x 10(-7), and 4.0 x 10(-7)m, respectively. High concentrations (> 0.1 mm) of histidine, leucine, methionine, alanine, cysteine, and aspartic acid also had an affinity for this system. Mutants lacking the general aromatic transport system were resistant to p-fluorophenylalanine, beta-2-thienylalanine, and 5-methyltryptophan. They mapped at a locus, aroP, between leu and pan on the chromosome, being 30% cotransducible with leu and 43% cotransducible with pan. Phenylalanine, tyrosine, and tryptophan were also transported by three specific transport systems. The apparent Michaelis constants of these systems were 2.0 x 10(-6), 2.2 x 10(-6), and 3.0 x 10(-6)m, respectively. An external energy source, such as glucose, was not required for activity of either general or specific aromatic transport systems. Azide and 2,4-dinitrophenol, however, inhibited all aromatic transport, indicating that energy production is necessary. Between 80 and 90% of the trichloroacetic acid-soluble pool formed from a particular exogenous aromatic amino acid was generated by the general aromatic transport system. This contribution was abolished when uptake was inhibited by competition by the other aromatic amino acids or by mutation in aroP. Incorporation of the former amino acid into protein was not affected by the reduction in its pool size, indicating that the general aromatic transport system is not essential for the supply of external aromatic amino acids to protein synthesis.

Genetic analysis of a large-cell, radiation-resistant strain of Escherichia coli

A genetic analysis of Escherichia coli P6, a large-cell, radiation-resistant strain of E. coli, established that it originated as the result of a mutational event. The gene responsible for the complex P6 phenotype was located at 61 +/- 0.5 min on the E. coli linkage map. The close resemblance of conjugal and transductional recombinants to one or the other parent without indication of an intermediate class suggests that only a single gene may be involved.

Role of pili in bacterial conjugation

We describe techniques for isolating individual pairs of mating Escherichia coli and observing them under the light microscope. Some pairs achieved close cell-to-cell contact, whereas others remained loosely connected by invisible connections which may be F pili. After 30 min of mating, the pairs were separated and allowed to grow into clones. That many exconjugants derived from "loose"-mating pairs produced recombinants suggests that F pili are involved in the transfer of genetic material. The frequency of formation of recombinants from "close"-mating pairs, however, was significantly higher than that from loose-mating pairs, indicating that a close cell-to-cell contact facilitates chromosome transfer. Death rates of exconjugants from close pairs were also higher than those from loose pairs. Hfr x F(-) matings produced higher death rates than F(+) x F(-) matings. Male cells were found capable of transferring genetic markers to two F(-) cells simultaneously. We conclude that F pili play at least three roles in mating: (i) they initiate contacts between mating pairs; (ii) they facilitate the transfer of genetic material; and (iii) they draw mating cells into a close contact which increases the fertility of the union.

Transketolase mutants of Escherichia coli

Transketolase mutants have been selected after ethyl methane sulfonate mutagenesis of Escherichia coli. These strains are unable to grow on any pentose and, in addition, require a supplement of aromatic amino acids or shikimic acid for normal growth on any other carbon source. Revertants are normal in both respects and also contain transketolase. Transketolase mutants do not require exogenous pentose for growth. Preliminary genetic mapping of the locus is presented.

Early stages of conjugation in Escherichia coli

We initiated these studies to learn more about the initial events during bacterial conjugation and to optimize conditions for their occurrence. We found that cells in donor cultures grown anaerobically prior to mating have (i) a higher mean number of F pili per cell, (ii) longer F pili, (iii) a higher probability of forming specific pairs with F(-) cells, and (iv) a faster rate of initiation of chromosome transfer than cells grown aerobically. The growth medium for the donor culture also influences these same parameters: a rich medium is superior to a completely synthetic medium. Starvation of donor cells in buffered saline or for a required amino acid results in (i) a loss of F pili, (ii) a loss in the ability of donor-specific phages to adsorb, (iii) a loss of ability to form specific pairs with F(-) cells and to yield recombinants, and (iv) an increase in recipient ability. These changes occur as a function of starvation time, and at rates which are dependent on the conditions of prior growth and starvation of the donor culture. Either treatment provides a rapid method for the production of F(-) phenocopies from donor cultures. Resynthesis of F pili by cells within a starved donor culture commences very soon after restoration of normal growth conditions, but full restoration of donor ability, as measured by recombinant yield, occurs at a slower rate. We found, along with other investigators, that F pili are essential for specific pair formation. We also found, however, that the presence of F pili is not sufficient for display of donor ability, nor is the absence of F pili enough for cells to exhibit recipient ability. This suggests, therefore, that one or more components, in addition to F pili, are necessary for the conversion of specific pairs to effective pairs (or for chromosome mobilization, or both) and for preventing donor cells from acting as recipients. On the basis of our results, we suggest optimal conditions for achieving high mating efficiencies.

Conjugation in Escherichia coli K-12 and its modification by irradiation

The steps of normal bacterial conjugation (union, transfer, integration and segregation) are described in analytical terms. Only two parameters are utilized: nu(mt) (0), the probability of interruption of transfer of the male chromosome per unit chromosomal distance; and nu(r) (0), the probability per unit chromosomal distance of a recombinational event. Experimentally these two parameters have the same value (0.06 min(-1) or 10(-6) per nucleotide pair). Irradiation of the donor parent prior to mating increases the transfer parameter (nu(mt) = nu(mt) (0) + sigma(mt)D) and a complete description of the radiation response of recombinant production is obtained by a consideration of the single parameter sigma(mt). Irradiation of the recipient parent prior to mating increases the recombination parameter (nu(r) = nu(r) (0) + sigma(r)D) and a complete description of the radiation response of recombinant production is obtained by the addition of the parameter sigma(r). Experimentally sigma(mt) and sigma(r) are found to have the same value, approximately 0.004 krad(-1) min(-1) for X-irradiation. It is thus possible to describe mathematically the behavior of the unperturbed mating system by a single parameter nu(0); a single additional parameter sigma is adequate to describe the behavior of the system when either parental type is irradiated prior to mating. The unexpected observation that nu(mt) and nu(r) have the same value suggests that common molecular mechanisms are involved in the transfer and integration steps.

Identification of closely linked loci controlling ultraviolet sensitivity and refractivity to colicin E2 in Escherichia coli

Mutants (phenotypic symbol Ref-II) refractory to colicin E2 have been isolated in several strains of Escherichia coli K-12, and a refII locus has been mapped 1 to 2 min counter clockwise to thr. A small number of Ref-II mutants are also ultraviolet (UV)-sensitive and the uv(s) locus in one such strain has been mapped close to the refII locus near thr. The Ref-II mutation alone does not affect recombinant formation in F(-) strains, but the Ref-II, UV(s) strains behave in many respects like Rec(-) mutants, giving reduced recombination frequencies in crosses with male strains. It is suggested that the refII and uv(s) loci correspond to closely linked if not identical genes, concerned in some way in the activity of one or more deoxyribonucleases, and that the Ref-II, UV(s) mutants arise as the pleiotropic expression of a single gene or of a deletion or polar mutation affecting linked genes.

Effects of temperature, agitation, and donor strain on chromosome transfer in Escherichia coli K-12

Recombinant production in Escherichia coli K-12 can be described by three parameters: (i) the distance x of a selected male marker from the donor origin; (ii) the gradient constant k (the probability of interruption of the donor chromosome per unit distance during transfer into a recipient cell); and (iii) the extrapolate number A (the probability that a donor cell will produce a recombinant inheriting the donor marker contiguous with the origin). It is usually assumed that chromosomal distances can be measured by marker entry times, i.e., that the velocity of chromosome transfer v is constant along the chromosome. The dependencies of k, A, x, and v on temperature, agitation during mating, and donor strain were studied. The transfer velocity of the HfrH chromosomal region from the origin to his increases 15-fold between 16 and 43 C, and the chromosomal regions studied have the same temperature dependence that was found for the separate transfer velocities of the O-trp and trp-his regions. These data and radiation studies on chromosome transfer indicate that, at a given temperature, chromosomal transfer velocity varies by less than 10% as the distance of any given region from the origin increases. The gradient constant k is temperature-independent between 20 and 45 C if mating times at different temperatures are inversely proportional to the chromosome velocities; also, k is insensitive to agitation during mating and is not decreased by mating on membrane filters. However, the extrapolate number A is highly temperature-dependent, having its maximum value between 30 and 38 C. These results suggest that the spontaneous interruption of transfer which produces the gradient of transfer is a property of the chromosome itself and not of the fragility of the connection between mating cells.

Dark recovery processes in Escherichia coli irradiated with ultraviolet light. I. Effect of rec mutations on liquid holding recovery

We have examined various derivatives of Escherichia coli K-12 for liquid holding recovery, a type of recovery originally observed in E. coli B irradiated with ultraviolet light. Although most of the K-12 derivatives tested showed relatively little or no recovery under our conditions, four of the six independent rec(-) mutants examined, those carrying recA1, rec-12, recA13, and rec-56, respectively, displayed marked recovery. These mutants are distinguished from rec(+) strains by their increased sensitivity to ultraviolet radiation and decreased ability to undergo genetic recombination. Two of them have also been reported to release large amounts of their deoxyribonucleic acid as acid-soluble material, especially after irradiation. None of the three uvr(-) mutants examined, containing uvrA6, uvrB5, or uvrC34, showed comparable liquid holding recovery. The one rec(-) uvr(-) derivative tested, carrying recA13 and uvrA6, did not appear to undergo liquid holding recovery, although recA13 uvr(+) strains did. Genetic analysis of one strain, a recA13 mutant, indicated that all the rec(+) derivatives obtained from it by conjugation, transduction and reversion, had lost the property of showing liquid holding recovery. From these results, we conclude that in E. coli K-12 the expression of liquid holding recovery depends upon certain rec(-) mutations.

Genetic mapping of loci for glucose-6-phosphate dehydrogenase, gluconate-6-phosphate dehydrogenase, and gluconate-6-phosphate dehydrase in Escherichia coli

The loci on the Escherichia coli genome of mutations affecting the constitutive enzymes glucose-6-phosphate dehydrogenase (zwf) and gluconate-6-phosphate dehydrogenase (gnd), and the inducible enzyme gluconate-6-phosphate dehydrase (edd), were determined by conjugation and transduction experiments, chiefly by three-factor crosses. They are in the same region of the chromosome, and their order is gnd-his-(edd, zwf)-aroD; gnd and his are cotransduceable, as are zwf and edd. The position of gnd in Salmonella typhimurium was shown to be similar to that in E. coli.