Molecular studies on entry exclusion in Escherichia coli minicells

Determinants of entry exclusion within Eex and TraG are cytoplasmic

We report here functional and topological analyses of TraG and Eex, the donor and recipient cell inner membrane proteins that mediate entry exclusion in the SXT/R391 family of integrative conjugative elements. We found that the exclusion-determining regions of the Eex variants EexS (SXT) and EexR (R391) are located in distinct yet overlapping regions of the proteins. Unexpectedly, the carboxyl-terminal regions of TraG and Eex, which contain the residues essential for exclusion activity and specificity, were found to localize in the cell cytoplasm. These observations suggest that complex topological rearrangements of conjugative proteins must occur during mating to enable these domains to interact.

Relationship between the proteins encoded by the exclusion determining locus of the IncI plasmid R144 and the cellular localization of these proteins in Escherichia coli K-12

A region of the IncI plasmid R144, determining and controlling exclusion (exc), codes for two proteins, designated 13K and 19K after their apparent molecular weights (respectively 13,000 and 19,000). Both proteins were simultaneously affected by various mutations that resulted in exclusion deficiency. In this paper the relationship between these proteins as well as their cellular location is reported. We found no indications that the 19K protein is a precursor form of the 13K protein. Analysis of gene products of recombinant plasmids carrying exc as well as of several derivatives, however, provided a strong indication that the proteins result from overlapping genes. Besides, evidence was obtained that the 19K protein is essential for exclusion. Localization studies revealed that this protein exists in a membrane-bound form, associated at the periplasmic side of the inner membrane, and in a soluble form residing in the cytoplasm.

Exclusion in IncI-type Escherichia coli conjugations: the stage of conjugation at which exclusion operates

The stage at which exclusion operates in matings between donors belonging to the I-type incompatibility group (IncI) was investigated. Mating between Escherichia coli cells harbouring the I-type plasmid R144 and E. coli cells harbouring the R144-derived recombinant plasmid pRAH308, which causes a hundredfold exclusion, was performed on a membrane filter to test whether mating aggregate formation was disturbed. Besides, level and kinetics of the formation of mating aggregates in mixtures of R144+ donor cells and recipient cells carrying plasmid pRAH308 (exclusion-proficient) was compared with the aggregate formation in mixtures of the donor cells and exclusion-deficient recipient cells. Results from these experiments revealed that the exclusion by pRAH308 does not operate at the level of aggregate formation, but acts at the stage of DNA transfer. The exclusion phenomenon by the recombinant plasmid pRAH308 appeared to be representative for exclusion caused by plasmid R144, since essentially identical results were obtained if plasmid R144 was used as exclusion-determining factor.

Cloning of an exclusion-determining fragment of the IncI plasmid, R144

By cloning a distinct 8 MDa fragment of the IncI plasmid, R144, in the vector pACYC184, two recombinant plasmids were isolated. In these plasmids, pRAH303 and pRAH308, the inserted fragment was in opposite orientations. Both plasmids when present in a recipient strain caused a conjugation-specific exclusion in crosses with donor cells carrying the IncI plasmid R144. Some derivatives of the recombinant plasmids in which parts were deleted, or in which Tn5 transposons were inserted, appeared to be exclusion negative. Analysis in minicells of the gene products of such plasmids together with those of the original recombinant plasmids revealed that the presence of two proteins, with apparent molecular weights of 13,000 and 19,000 Da could be correlated with the exclusion phenomenon.

Escherichia coli Hfr-DNA degradation in endonuclease I-deficient minicells

[3H]Thymidine (dThd)-labelled Hfr DNA was transferred by conjugation into Escherichia coli F- minicells harvested from an endonuclease I-deficient (endI-) strain and its iosgenic wild type (endI+) parent. The susceptibility of this DNA to attack by DNAase was examined. The kinetics of in vivo conversion of [3H]dThd-labelled DNA into acid soluble radioactivity was examined. This activity, attributed to exonuclease action was the same for both strains. Contribution of endonuclease I was measured by an analysis of changes in weight-average (Mw) and number-average (Mn) molecular weight distribution of DNA molecules recovered from minicells. Reduction in Mw was greater in the endI-strain. The ratio Mn/Mw changed drastically during the incubation period of endI- minicells, but remained unchanged in the endI+ strain. These experiments suggest that the presence of the endI- mutation in minicell-producing strain chi1268 leads to a greater loss in M2 of Hfr DNA conjugally transferred into the minicells.

Relation between F, R1, R100 and R144 Escherichia coli K-12 donor strains in mating

E. coli K-12 recipient mutants defective in conjugation with a F donor (ConF- mutants) were tested for recipient ability with donor strains carrying a R1drd19, R100-1 or R144drd3 plasmid. It appeared from these tests that F and R1 donor strains are closely related in mating in that they both, unlike the R100 and R144 donor, use the recipient outer membrane protein pOmpA for the initial steps of the mating process. The isolation of recipient mutants defective in conjugation with a R100 donor (ConR100- mutants) yielded some mutants that were defective in crosses with F, R1 and R100 donors. This result was taken as evidence that F, R1 and R100 donors share some property in their mating interaction with E. coli K-12 recipient strains. A R144 donor is completely different from F, R1 and R100 donors in initial mating interactions with a recipient.

Effect of entry exclusion on mating aggregates and transconjugants

Mating aggregates during conjugation directed by an F-like R factor in Escherichia coli were measured as the number of Lac+-Lac- sectored colonies present in a mating mixture. There is a high degree of correlation between the concentration of transconjugants produced in a mating mixture and the concentration of mating aggregates observed at several different concentrations of donor and recipient cells. The mating aggregates are sex pilus specific as demonstrated by the ability of donor-specific ribonucleic acid phage MS-2 to decrease both mating aggregates and transconjugants in a mating mixture. During entry exclusion by either a derepressed or a repressed F-like R factor, isogenic to the superinfecting R factor except for a resistance determinant, the number of transconjugants was markedly reduced, but the number of mating aggregates was not decreased. Entry exclusion by F-Gal toward the donor HfrH resembled that of the F-like R factor in that there was a reduction in the number of recombinants but no significant decrease in mating aggregates. These results suggest that entry exclusion inhibits conjugation at a stage after the formation of mating aggregates.

Molecular cloning of DNA from F sex factor of Escherichia coli K-12

We describe the molecular cloning of various DNA segments generated by partial EcoRI endonuclease digestion of the sex factor F. These segments have been analyzed by agarose gel electrophoresis of EcoRI digests and were arranged in a series of overlapping fragments using the EcoRI fragment map of F established by H. Ohtsubo and E. Ohtsubo. The clones isolated demonstrate one or more of the following F-specified functions: inhibition of female-specific phage (T7) multiplication, formation of F pili, surface exclusion, or immunity to lethal zygosis. These properties are discussed in terms of the EcoRI fragments of F that specify them.

Mating signal and DNA penetration deficiency in conjugation between male Escherichia coli and minicells

Strain OU122, a dnaB mutant of HfrH which stops vegetative DNA synthesis immediately when the temperature is shifted to 43 degrees, was mated at 43 degrees with minicells with and without 1 mM Zn2+. Synthesis of DNA was detected in the mating mixture containing minicells derived from either Fchi925 or F+chi115 cells, but only a small amount was detected in the mixture containing 1 mM Zn2+ which inhibits the formation of mates (previously called mating pairs). The supernatant liquid from cell cultures did not induce DNA synthesis, suggesting that DNA synthesis was not stimulated by diffusible molecules. Additional experiments showed that Zn2+ inhibited DNA synthesis associated with DNA transfer but did not inhibit the DNA transfer that had already been initiated. Thus, the stimulation of DNA synthesis observed required physical interaction of cells and F pili. Reisolated minicell exconjugants derived from the cross OU122 X F--chi925 minicells were shown to contain transferred DNA in contrast to either the cross OU122 X F--chi925 minicells in 1 mM Zn2+, or the cross OU122 X F+chi1115 minicells. Thus, F+chi1115 minicells retained the property of surface exclusion at 43 degrees.

Role of the cell surface in bacterial mating: requirement for intact mucopeptide in donors for the expression of surface exclusion in R+ strains of Escherichia coli

Two derivatives of the F-like R factor R1drd19 carrying mutually exclusive resistance determinants were used to study the role of the mucopeptide in the expression of conjugal functions. The use of metabolically active penicillin spheroplasts in R+ times R- matings had no effect on the ability of the cells to donate or accept a plasmid. However, in R+ times R+ matings it was found that surface exclusion was totally abolished if the donor, but not the recipient, was a spheroplast. This result implies that the traS gene, expressed by the excluding plasmid, is dependent for its action on an intact mucopeptide layer in the donor cell, and that this interaction is independent of the transfer ability of the excluding plasmid.

Isolation of a hybrid F' factor-carrying Escherichia coli lactose region and Salmonella typhimurium histidine region, F42-400 (F' ts114 lac+, his+): its partial characterization and behavior in Salmonella typhimurium

Episome F' ts114 lac+ (F42-114) was transferred into Salmonella typhimurium carrying an F'his+ (FS400) episome, and fused episome F' ts114 lac+, his+ (F42-400) was obtained. Episome F42-400 could be transferred to S. typhimurium, Escherichia coli and Klebsiella pneumoniae. Identification of the episome was based on: (i) temperature sensitivity of the Lac+ and His+ phenotypes; (ii) the fact that F- segregants, obtained after temperature curing or acridine orange curing, were simultaneously Lac- and His-; and (iii) linkage of lac+ with his+ in episomal transfers to E. coli and S. typhimurium. The frequency of episome transfer was influenced by the genotype of the donor. Plasmid LT2, prevalent in S. typhimurium LT2 strains, was suggested to be responsible for the low fertility of S. typhimurium donors. Episome F42-400 was capable of chromosome mobilization, and the extent of chromosome mobilization was not influenced by the presence or absence of the histidine region on the donor chromosome. Growth in a defined medium with acridine orange was able to cure F42-400. The frequency of curing was increased (the frequency of His+ cells was 0.0001%) if the cells were grown at 40 C in the presence of acridine orange. Selection for temperature-resistant Lac+, His+ derivatives in a strain without histidine deletion yielded Hfr strains. However, similar and stronger selections in strains without the chromosomal histidine region failed to yield Hfr strains. Our inability to obtain Hfr's in strains without the chromosomal histidine region was explained by assuming that the episome F42-400 has lost the F sites involved in integration into the S. typhimurium chromosome.

Restoration of F' superinfection inhibition in a DnaB mutant of Escherichia coli upon construction of heterozygous DnaB merodiploids or P1 lysogens carrying a dnaB analogue

F-prime derivatives of the Escherichia coli strain CR34 bearing the thermosensitivity mutation dnaB43 display low levels of plasmid-determined superinfection inhibition in conjugational crosses at 30 C. Salt-mediated phenotypic suppression of this temperature sensitivity fails to restore normal levels of inhibition, indicating its alteration is not a secondary effect of dnaB43 a-tion on growth or deoxyribonucleic acid syntheiss. Superinfection inhibition is fully restored in mutant cells made merodiploid for the dnaB region by introduction of the F' dnaB-+ plasmid F134-1. dnaB43-bearing strains lysogenized with P1 phage contribution dnaB-analogue protein show eight to nine times more superinfection inhibition than do the same cells carrying P1 prophage repressed dnaB-analogue protein production. Taken together, this evidence suggests a direct causal relationship between dnaB43 and the altered superinfection inhibition phenotype.

dnaB gene of Escherichia coli K-12 affects superinfection inhibition between F' plasmids

F' Escherichia coli K-12 strains bearing the chromosomal mutation dnaB43 offer significantly less resistance to the conjugational introduction of a second F' plasmid than do nonmutant strains. Both the entry exclusion and incompatibility components of superinfection inhibition are altered. This action of dnaB43 occurs regardless of the presence of a recA-minus mutation in matings in liquid cultures and on membrane filters and is not limited to a particular set of F' plasmids. These effects are co-transducible by phage P1 with the temperature sensitivity conferred by dnaB43. The effects also occur with a strain carrying dnaB107. In the double F' strains that arise, the two plasmids exist as units autonomous of one another and the chromosome.

DNA degradation in minicells of Escherichia coli K-12. II. Effect of recA1 and recB21 mutations on DNA degradation in minicells and detection of exonuclease V activity

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 transfereed 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 R 64-11 or lambdadv, 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.

Interactions between the surface exclusion systems of some F-like plasmids

Four surface exclusion systems have been identified amongst a group of F-like plasmids inE. coli: SfxI(F), SfxII(ColV2 and R538-1fin−), SfxIII(ColVBtrpand R1-19) and SfxIV(R100-1 and R136fin−). None of these systems was expressed in stationary phase cells or, except for ColVBtrp, duringfin+transfer inhibition, showing that the surface exclusion gene(s) is usually co-controlled with the transfer genes.

Recipient cells carrying two plasmids specifying different surface exclusion systems did not always express both of these: the overall pattern suggested that the four systems and/or their sites of action are related. There was no surface exclusion between donor cells carrying two plasmids determining different surface exclusion systems and recipient cells carrying a plasmid determining either one of these. Our hypothesis to explain this and other results is that surface exclusion prevents interaction between the tip of the pilus on the donor cell and a receptor site on the recipient cell surface. Pili (probably mixed) with two types of tips would be present on cells carrying two different plasmids, the one unresponsive to the single surface exclusion system of the recipient cells allowing transfer of both plasmids.

Mapping loci for surface exclusion and incompatibility on the F factor of Escherichia coli K-12

A series of Hfr deletion strains carrying deletions extending different distances into the integrated F factor have been used to map loci for surface exclusion (traS) and for incompatibility (inc) on the Escherichia coli K-12 sex factor F. traS mapped between traG and traD. It forms a part of the large operon, including all the known transfer genes except traJ, and is co-controlled with these. The product of traS is not required for formation of the F pilus. inc mapped between the phi(R) (11) locus and the origin of transfer; it is therefore one of the earliest loci transferred during conjugation.

F factor-mediated immunity to lethal zygosis in Escherichia coli K-12

Recipient (F(-)) cells of Escherichia coli are sensitive to an excess of Hfr donor cells. This phenomenon of lethal zygosis is associated with conjugation and is observed as a continuous fall in F(-) viable cells during liquid mating, or as inhibition of F(-) growth on solid media. One class of survivors, which arose in the zones of inhibition on solid media, was no longer sensitive to lethal zygosis and exhibited the following properties: sensitivity to male-specific phage, donor ability, and surface exclusion. Since these characteristics were sensitive to acridine orange treatment, the strains carry an F factor extrachromosomally. They are, however, defective in some way since they retain sensitivity to female-specific phage. Temporary sensitivity to lethal zygosis in these and in standard F(+) strains can be induced by the formation of F(-) phenocopies. We have suggested that there is an immunity to lethal zygosis (Ilz(+)) associated with the F factor and discuss the results in terms of this hypothesis.

Conjugal deoxyribonucleic acid replication by Escherichia coli K-12: stimulation in dnaB(ts) donors by minicells

R64-11(+) donor cells that are thermosensitive for vegetative DNA replication will synthesize DNA at the restrictive temperature when recipient minicells are present. This is conjugal DNA replication because it is R64-11 DNA that is being synthesized and there is no DNA synthesis if minicells that cannot be recipients of R64-11 DNA are used. The plasmid DNA present in the donor cells before mating is transferred to recipient minicells within the first 20 min of mating, but additional copies of plasmid DNA synthesized during the mating continue to be transferred for at least 90 min. However, the transfer of R64-11 DNA to minicells is not continuous because the plasmid DNA in minicells is the size of one R64-11 molecule or smaller, and there are delays between the rounds of plasmid transfer. DNA is synthesized in minicells during conjugation, but this DNA has a molecular weight much smaller than that of R64-11. Thus, recipient minicells are defective and are not able to complete the synthesis of a DNA strand complementary to the single-stranded R64-11 DNA received from the donor cell.

Membrane attachment of R-factor deoxyribonucleic acid in compatible and incompatible cell pairs following conjugation

Superinfection of an Escherichia coli R(+) strain with a closely related R-factor was associated with the accumulation of replicative intermediates of the superinfecting deoxyribonucleic acid. This deoxyribonucleic acid remains associated with a rapidly sedimenting cell component.

Effect on exclusion of alterations to the sex pilus

Chromosomal genes from an Hfr donor, dependent for their transfer upon the integrated F factor, were not excluded by an F(+) recipient when the donor also carried an F-like R factor, and its sex pili contained, in addition to F pilin, another pilin of a different specificity.