Expression of a mutation affecting F incompatibility in the integrated but not the autonomous state of F.

Previously we have described a mutant Hfr strain in which incompatibility between the integrated F factor and an autonomous F-prime (F') factor was abolished. The mutation (inc) was located in the integrated F factor. F-prime factors isolated from the mutant Hfr strain have the same incompatibility behavior as those isolated from normal Hfr strains. Reintegration of these F' factors into the chromosome restores the Inc- phenotype characteristic of the mutant Hfr. The inc mutation thus affects incompatibility between integrated F and autonomous F(Fi-Fa incompatibility) but not incompatibility between two autonomous F factors (Fa-Fa incompatibility). The implications of this finding for the mechanism of plasmid incompatibility are discussed.     

Evolution of a Site of Specific Genetic Homology on the Chromosome of Escherichia coli

Integration of the factors F(v) and F into the chromosome of a substrain of Escherichia coli K-12 has been studied. The F(v) factor is a fertility factor derived from Col V, lacking the ability to govern the production of colicin V. The derivatives of an Hfr(v) (Hfr isolated from a V colicinogenic parent) strain, PK2 (initially isolated from C600 V(+)), were shown to retain a unique bidirectional sex factor affinity locus between recA and pheA. This site shows no affinity for the E. coli K-12 F factor as shown by inability to isolate Hfr strains with origins in this region from a parental strain containing a cytoplasmic F factor. However this area exhibits two regions of homology to the V colicinogenic factor. One gives rise to Hfr(v) strains identical to the original Hfr(v) strain, PK2, with an origin and polarity of transfer designated pheA-CC injecting markers in the order pheA-his-trp-pro. The second gives rise to strains apparently originating at the same site but with reverse polarity designated recA-C, transferring markers in the order recA-thyA-str-xyl. For strains possessing the F(v) factor only the second homology is apparent. A model for the evolution of these strains is presented.     

Description of an incompatibility mutant of Escherichia coli

A mutant Hfr strain of Escherichia coli which has an impaired incompatibility function but is normal for other F factor functions has been isolated. This Inc(-) Hfr permits the maintenance and transfer of both the integrated F factor and an F' factor. F' factors have been isolated from the integrated F factor of the Inc(-) Hfr strain. When these episomes were tested in matings with Hfr or F' strains, they did not differ in any observed way from wild-type F' factors.     

Conjugation in Escherichia coli: Failure to Confirm the Transfer of Part of Sex Factor at the Leading End of the Donor Chromosome

Experiments were carried out attempting to determine whether part of sex factor is transferred at the leading end of the Hfr chromosome during conjugation. In the first experiment, an analysis was made of the donor properties of recombinant strains which had inherited the terminal but not the proximal marker from an Hfr. Secondly, recombinants integrating an extremely proximal marker from an Hfr were examined for the inheritance of a sex factor affinity locus adjacent to this marker. In the third experiment, proximal transfer of the wild-type allele of a temperature-sensitive sex factor mutation was looked for, using as recipient a temperature-sensitive Hfr strain, and as donor a wild-type Hfr isogenic with respect to the site of sex factor integration. In none of these experiments could the presence of sex factor material at the leading end be demonstrated. The results do not rule out the possibility that part of F is transferred proximally but only integrated at a very low frequency. They do, however, conflict with certain findings of other authors which, in the past, have been taken as evidence for the transfer of part of F at the leading end.     

A Comparative Study of Two F-like Colicin Factors, ColV2 and ColV3, in Escherichia coli K-12

Of three colicin factors, each determining the synthesis of a colicin V in three wild-type Escherichia coli strains studied, two were shown to have sex-factor activities. In E. coli K-12, these activities resembled those of the F sex factor (including rapid and efficient self-transmission in exponentially growing cultures, adsorption of "male-specific" ribonucleic acid phage, production of "female phenocopies," elimination by acridine orange, and chromosomal transfer dependent upon recombination with host bacterium) and differed in this way from those of the colicin I sex factor (ColI). The two V factors, ColV2 and ColV3, differed in their efficiency of plating male-specific phage and in the pattern of transfer of chromosomal markers. Furthermore, although neither factor could stably coexist with F within the same cell, they showed markedly different exclusion effects. In general, ColV2 excluded F and ColV3 was excluded by F, irrespective of which sex factor was preestablished in the cell. An exception to this was the ability of ColV2 to stabilize in any one of a series of Hfr strains, giving rise to strains which in the majority of cases showed normal Hfr and colicinogenic properties.     

Nitrosoguanidine Assay of Episomal Integration in Escherichia coli

Mutations affecting utilization of lactose and resistance to the male-specific phages f1, f2, and Qbeta tend to occur simultaneously more often than expected by chance in Hfr strains whose origin of transfer is close to the genes for lactose utilization, but not in F(+) strains. Strains derived from the Hfr, but exhibiting poor ability to transfer early chromosomal genes, may or may not show this comutation phenomenon. These results support the concept that the F factor is integrated into the Hfr chromosome during vegetative growth, but is autonomous in the F(+) strains and could serve as an assay for episomal localization.     

Genetic Factors Affecting Recovery of Nonpoint Mutations in the Region of a Gene Coding for Ornithine Transcarbamylase: Involvement of Both the F Factor in Its Chromosomal State and the recA Gene

Mutants of E. coli K12 that overproduce ornithine transcarbamylase can be identified in Car^- strains because they permit utilization of citrulline as a carbamyl phosphate source, due to reversal of the normal OTCase reaction; they are called Cut mutants (citrulline utilizers). Hfr strains that carry the F factor adjacent to argF (one of two duplicate genes that code for ornithine transcarbamylase in E. coli K12) yield more Cut mutants than do F⁺ or F^- strains, or Hfr strains in which the F factor is not adjacent to argF. When Hfr strains in which the F factor is integrated adjacent to argF are made recA, they yield few Cut mutants. Many of the Cut mutants recovered from one of the Hfr strains used in the investigation (Hfr P4X) are unstable; the properties of these unstable mutations suggest that they carry aberrations in the region of the argF gene. Thus, the increased yields of Cut mutants probably result from aberrations that occur when the F factor is integrated adjacent to argF. The nature of these aberrations is not yet known. The unstable Cut mutants are to a large extent stabilized by recA; such stabilization is one of the properties of duplications. Other data indicate that the aberrations may be more complex than simple gene duplications; in particular properties of segregants and some recombinants derived from unstable Cut mutants are most easily interpreted by assuming that segregation from, and possibly formation of, the unstable mutants occurs in several stages.     

Inhibition of Replication of an F′lac Episome in Hfr Cells of Escherichia coli

Hfr strains of Escherichia coli K-12 were found capable of accepting a F'lac episome during mating, with a frequency approximating that of F(-) strains. However, the F'lac episome was unable to replicate in the Hfr cells, and was diluted out during the growth of the culture. The lac(+) gene of the episome can be "rescued" by recombination into the host chromosome, as shown by the appearance of variegated recombinant colonies on a lactose-fermentation indicator medium. In recA Hfr strains, however, no lac(+) offspring were obtained in crosses with F'lac donors. The induced synthesis of beta-galactosidase in F'lac(+) x Hfr zygotes was studied. Rates of enzyme synthesis were approximately constant with respect to time as expected from unilinear inheritance of the F'lac episome. However, the rate of synthesis eventually increased, presumably due to integration of the lac(+) gene in some of the zygotes. In F'lac(+) x recA Hfr zygotes the rate of beta-galactosidase synthesis remained constant with respect to time, as expected.     

Characterization of Lethal Zygosis Associated with Conjugation in Escherichia coli K-12

When F(-) cells are mixed with an excess of Hfr cells there is a lethal event which results in a decrease in the number of F(-) survivors. We have described and discussed the parameters affecting this phenomenon of lethal zygosis, and these include the cultural conditions of both donor and recipient cells prior to mixing and the use of aeration throughout the period of the experiment. The absence of lethal zygosis with filtrates and supernatant fluids from donors suggests a dependence on direct cell-cell contact as found in conjugation. The phenomenon, which is normally observed in liquid media, also occurs on solid media, and use of these two methods has allowed examination of strains of different mating types. Whereas most Hfr strains capable of producing normal yields of recombinants showed killing activity, no F(+) and only one F' donor produced lethal zygosis. Only F(-) strains were sensitive to this phenomenon. The relationship between lethal zygosis and the various stages of conjugation is discussed.     

Suppression of initiation-negative strains of Escherichia coli by integration of the sex factor F.

Data are presented suggesting that the most critical factor determining whether an Hfr dnaAts strain can synthesize deoxyribonucleic acid and form colonies at temperatures that are nonpermissive for corresponding F- strains is neither the site of insertion of F nor the presence of additional mutations in the F particle or the chromosome; it is whether the particle is capable of autonomous replication at the temperature used. Consequently, suppression of the DnaA phenotype in Hfr strains occurs at 40 C but not, in most of them, at 42 C without the occurrence of additional mutations. The site of insertion of F may also be important since it is shown that in one Hfr dnaA strain partial suppression does occur at 42 C. In addition, it is shown that strains exhibiting suppression by integration of F at 40 C on minimal agar plates do not do so at this temperature on nutrient agar plates.     

Isolation and Characterization of a Recombination-Deficient Hfr Strain

The isolation of a rec(-) Hfr strain of Escherichia coli K-12 is described. The method used consisted of mating AB2463 F(-) Rec(-) His(-) Lac(-) with P4X6 Hfr Rec(+) His(+) Lac(+), selecting Rec(-) His(-) Lac(+) recombinants, and searching for Hfr strains. One Hfr rec(-) strain, no. 12, was used as donor in crosses with Rec(+) and Rec(-) recipients. Crosses with Rec(+) recipients are fertile, and those with Rec(-) recipients are almost infertile, the frequency of recombinants being 10(-2) to 10(-3) that found with Rec(+) recipients. The Rec(-) mutant marker is transfered to and integrated into Rec(+) recipients. Zygotic induction of prophage lambda is observed in crosses between two Rec(-) strains. In crosses of F(-) Rec(-) with Hfr Rec(-), the gradient of integration frequencies for markers progressively more distant from the origin is steeper than in the Rec(+) x Rec(-) or the Rec(-) x Rec(+) crosses.     

Isolation of High-Frequency Recombining Strains from Escherichia coli Containing the V Colicinogenic Factor

The isolation and characterization of high-frequency recombining strains from different Escherichia coli host cells containing either the F factor or the Col V factor are described. The strains (with one exception) formed from three of the V⁺ parents showed the same origin and polarity of transfer (xyl-arg-pro-trp-his-mal). The Hfr strains formed from the one remaining V⁺ and the F⁺ host cells showed a greater variety in their points of origin. In addition, several Hfr strains isolated from V⁺ parents lost the ability to produce colicin V. F_v⁺ segregants of these were isolated, and the F_v factors appeared to retain their preferential site for Hfr formation, but they lacked other propertes controlled by the Col V factor. Chromosomal integration of episomes and its relation to the fertility of F⁺ and V⁺ strains are discussed. Production of colicin V appeared to be uninfluenced by the state of the Col V factor within the cell.     

Isolation of Circular Deoxyribonucleic Acid from Salmonella typhosa Hybrids Obtained from Matings with Escherichia coli Hfr Donors

Heterozygous, partial diploid Salmonella typhosa hybrids obtained from matings with Escherichia coli K-12 Hfr strains were observed to contain supercoiled, circular deoxyribonucleic acid (DNA) when examined by the dye-buoyant density method. Examination of one such S. typhosa hybrid after its loss, by segregation, of the inherited E. coli genetic markers revealed a concurrent loss of its supercoiled circular DNA. Subsequent remating of this segregant with various E. coli Hfr strains resulted in the reappearance of the circular DNA. Molecular weight determinations of circular DNA molecules isolated from a number of S. typhosa partial diploid hybrids were made by sucrose density gradient ultracentrifugation and electron microscopy. These studies revealed a range of molecular sizes among the various hybrids examined, but each hybrid exhibited only a single characteristic size for its contained circular DNA. The range of size is consistent with the presence in each hybrid of a different length of E. coli chromosome. It was concluded that the E. coli Hfr genetic segments transferred to these S. typhosa hybrids were conserved, in the diploid state, in the form of supercoiled, circular DNA molecules.     

Specific Action of Sodium Dodecyl Sulfate on the Sex Factor of Escherichia coli K-12 Hfr Strains

A specific action of sodium dodecyl sulfate (SDS) on the sex (F) factor in the integrated state of Escherichia coli K-12 Hfr H strain is reported. Growth of Hfr cells in Penassay Broth containing SDS results in the elimination of part or all of the F factor, yielding low and nonfertile variants of defective Hfr type and F⁺ cells and also F⁻ derivatives. Appearance of such variants was generally observed after the culture reached stationary phase. The frequencies of F⁻ cells then increased. F⁻ cells were usually isolated as the major population among survivors. Some defective variants of Hfr cells with an intermediate fertility between standard Hfr and F⁺ cells had lost sensitivity toward the male-specific ribonucleic acid phage M12. Other defective Hfr variants with as much or less fertility than standard F⁺ cells had also all lost sensitivity to phage M12. On single-colony isolation, they segregated nonfertile female H cells which, when infected with F, could restore high fertility with oriented transfer of the chromosome the same as that of the original Hfr H. Also, sensitivity to phage M12 was regained. Female H cells were characterized as those lacking fertility but still retaining a small segment of F or sfa locus at the original part of the chromosome, where newly infected F could attach. Similar results were obtained with two other Hfr strains. A possible mechanism of the specific action of SDS is discussed.     

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     

Identification of a membrane protein associated with expression of the surface exclusion region of the F transfer operon.

Membrane preparations from radioactively labeled male and female strains of Escherichia coli K-12 were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. An intensely labeled band corresponding to a protein of molecular weight of 24,000 was readily apparent in preparations from Hfr and F-prime strains but not in those from female strains. When preparations from a series of Hfr strains containing transfer operon deletions were examined, presence of the band was found to be associated with retention of the region of the F transfer operon between ilzA and traD. Thus, the band ("protein S") appears to be the product of an F tra operon activity corresponding to traS (the gene for surface or entry exclusion), or an unknown gene in its vicinity. As predicted, protein S was subject to Fin+ control; only a faint band was detectable if the repressed plasmid R100 was also present in the F lac strain. A 24,000-dalton protein was also found in membrane preparations from strains carrying the derepressed plasmids R100-1 and R1-19 but not in those from strains carrying the repressed plasmids R100 or R1. Thus, the appearance of protein S in the membrane may be a general phenomenon resulting from transfer operon expression of F-like plasmids.     

Early transfer of genes determining transfer functions by some Hfr strains in Escherichia coli K12

Summary Sixty-eight Hfr strains were examined for their ability to transfer early in conjugation the transfer genes carried by the integrated sex factor. This was measured by mating these strains with F^- phenocopied recipient cultures of strains carrying transfer-deficient Flac ⁺ factors, and then measuring the ability of the recipient strains to transfer lac ⁺ to a further recipient strain. Most Hfr strains did not complement the missing transfer functions, though in some strains complementation was observed. It is concluded that on the sex factors of different Hfr strains either the site at which integration occurs or the origin of transfer must vary.     

Isolation and characterization of Hfr strains of Erwinia amylovora

Hfr strains (Hfr 159 and its derivatives, Hfr 160 and Hfr 161) were constructed from Erwinia amylovora ICPB EA178 by introducing an Escherichia coli F'his+ plasmid and then selecting for integration of F'his+ after treatment with acridine orange. The Hfr strains were relatively stable upon repeated transfers on nonselective media. Interrupted mating experiments and analyses of inheritance of unselected markers showed that his+ is transferred by Hfr 159 as the proximal marker at a relatively high frequency (about 5 x 10(-4) recombinants per input donor cell), followed by ilv+, orn+, arg+, pro+, rbs+, met+, trp+, leu+, ser+, and thr+ (not necessarily in that precise order). The donor strains, previously constructed in E. amylovora by integration of F'lac+ from E. coli transfer cys+ as the proximal marker followed by ser+. Further analysis of one of those earlier donor strains, Hfr99, showed that ser+ is followed by arg+, orn+, met+, pro+, leu+, ilv+, rbs+, his+, trp+, and thr+ (not necessarily in that precise order). Thus, the Hfr strains constructed by integration of F'his+ are different, in terms of origin and direction of transfer, from those derived from integration of F'lac+. The applicability of these Hfr strains to mapping the genes on the E. amylovora chromosome is indicated.     

Chromosomal Integration of F′ Factors in Recombination-Deficient Hfr Strains of Escherichia coli

In matings between F′ donors and recombination-deficient Hfr recipients, we isolated progeny which transferred both episomal markers and Hfr markers early and with high frequency. A number of these progeny had two integrated sex factors. Investigation of these double Hfr strains showed that the F′ nearly always integrated in a homologous region of the chromosome. In any particular mating system integration was specific as to location and direction of chromosome transfer.     

IS2-IS2 and IS3-IS3 relative recombination frequencies in F integration

The relative integrative recombination frequencies of the F plasmid IS2 and IS3 elements were determined at two Escherichia coli K-12 chromosomal sites by hybridization analysis of Hfr DNA. The sequence organizations of the independent Hfr strains formed by F integration atα₃β₃ indicate that the relative recombinational frequencies at the two F plasmid IS3 elementsα₁β₁ and α₂β₂ are not significantly different. A comparison of the relative recombinational frequencies of the IS2 and IS3 elements of F was provided by analysis of DNA from Hfr strains having F integrated betweenlacandproC (i.e., at the IS2 or at the IS3 element (α₅β₅) located in this region). No instances of F integration atα₅β₅ were detected, indicating that integrative recombination at IS2 is significantly more frequent than at IS3 in this chromosomal region.