Reinitiation of chromosome replication in the presence of chloramphenicol under an integratively suppressed state by R6K

Novel alleles of the Escherichia coli dnaA gene are defective in replication of pSC101 but not of oriC

Five novel alleles of the Escherichia coli dnaA gene that were temperature sensitive in maintenance of pSC101, a plasmid that is dependent on this gene for replication, were isolated. Nucleotide sequence analysis revealed that four of the five alleles arose from single base substitutions, whereas the fifth contained three base substitutions, two of which were silent. Whereas all five alleles were temperature sensitive in vivo for pSC101 maintenance, genetic and biochemical characterization indicated that only two were defective in replication from the chromosomal origin, oriC. As previously characterized mutations are defective in replication for both pSC101 and oriC, the dnaA mutations specifically defective in pSC101 maintenance represent a novel class. We speculate that one or more of these pSC101-specific mutants are defective in interaction with pSC101 RepA protein, which is also required for initiation of plasmid DNA replication.

Cloning and characterization of dnaA(Cs), a mutation which leads to overinitiation of DNA replication in Escherichia coli K-12

The product of the dnaA gene is essential for the initiation of chromosomal DNA replication in Escherichia coli K-12. A cold-sensitive mutation, dnaA(Cs), was originally isolated as a putative intragenic suppressor of the temperature sensitivity of a dnaA46 mutant (G. Kellenberger-Gujer, A. J. Podhajska, and L. Caro, Mol. Gen. Genet. 162:9-16, 1978). The cold sensitivity of the dnaA(Cs) mutant was attributed to a loss of replication control resulting in overinitiation of DNA replication. We cloned and sequenced the dnaA gene from the dnaA(Cs) mutant and showed that it contains three point mutations in addition to the original dnaA46(Ts) mutation. The dnaA(Cs) mutation was dominant to the wild-type allele. Overproduction of the DnaA(Cs) protein blocked cell growth. In contrast, overproduction of wild-type DnaA protein reduced the growth rate of cells but did not stop cell growth. Thus, the effect of elevated levels of the DnaA(Cs) protein was quite different from that of the wild-type protein under the same conditions.

The Hfr status of Pseudomonas aeruginosa is stabilized by integrative suppression

A temperature-sensitive mutant (dna-11) with the phenotype of a mutant defective in the initiation of DNA replication, was isolated from an Hfr-like FP2 donor of Pseudomonas aeruginosa. Reversion of its temperature-sensitive character was achieved by integrative suppression rather than by backmutation or an additional suppressor mutation. The dna-11 mutant proved to be helpful in stabilizing the Hfr status of the original host.

Host participation in plasmid maintenance: dependence upon dnaA of replicons derived from P1 and F

Nonparticipation of the bacterial dnaA gene in plasmid replication has been assumed to be the general rule. In conditional dnaA mutants of Escherichia coli, only plasmid pSC101 has been shown to have a dnaA requirement. Experiments with dnaA null mutants of E. coli, presented here, show that dnaA plays a critical and direct role in the replication of miniplasmids derived from P1 and F as it does in the initiation of bacterial replication. Evidence is also presented for the existence of a dnaA-independent secondary replicon of P1 that is able to drive bacterial chromosome replication but is inadequate to support the maintenance of P1 as a plasmid in E. coli.

Evidence for positive regulation of plasmid prophage P1 replication: integrative suppression by copy mutants

Like low-copy-number plasmids including P1 wild type, multicopy P1 mutants (P1 cop, maintained at five to eight copies per chromosome) can suppress the thermosensitive phenotype of an Escherichia coli dnaA host by forming a cointegrate. At 40 degrees C in a dnaA host suppressed by P1 cop, the only copy of P1 is the one in the host chromosome. Trivial explanations of the lack of extrachromosomal copies of P1 cop have been eliminated: (i) during integrative suppression, the P1 cop plasmid does not revert to cop+; (ii) the dnaA+ function of the host is not required to maintain P1 cop at a high copy number; and (iii) integrative recombination does not occur within the region of the plasmid involved in regulation of copy number. Since there are no more copies of the chromosomal origin (now located within the integrated P1 plasmid) than in a P1 cop+-suppressed strain, the extra initiation potential of the P1 cop is not used to provide multiple initiations of the chromosome. When a P1 cop-suppressed dnaA strain was grown at 30 degrees C so that replication could initiate from the chromosomal origin as well as from the P1 origin, multicopy supercoiled P1 DNA was found in the cells. This plasmid DNA was lost again when the temperature was shifted back to 40 degrees C.

Isolation and characterization of a higher-copy-number mutant of plasmid R6K

A stable copy-number mutant (pNH601) of plasmid R6K was isolated by selection for increased resistance to ampicillin determined by this plasmid. The size of the mutant plasmid was found to be unchanged (26 Mg/mol) but it is present in 27 copies of pNH601 per E. coli K-12 chromosome which represents a two-fold increase of R6K copy number value. The following genetic properties of pNH601 are reported and compared with those of R6K: conjugative transfer, fertility inhibition of plasmids belonging to other incompatibility groups, incompatibility with plasmid R485 under both non-selective and selective conditions and the integrative suppression of the dnaA ts mutation. The mutant plasmid pNH601 was found to be different from the original R6K in most of these properties.

Host components required for the replication of the resistance plasmid R124 and a copy mutant derivative

The replication of R124, and a copy mutant derivative of it, was measured with respect to dependence on the host DnaA, DnaB, DnaC, DnaE, DnaG, and PolA gene products. Both plasmids replicated under conditions where the DnaA gene product was inactivated or where the polymerising activity of the PolA gene product was reduced. In contrast, neither plasmid replicated to any appreciable extent, if the DnaB, DnaC, DnaE or DnaG gene products were inactivated. R124 integratively suppressed the lesion of the dnaA mutant but the copy mutant derivative had only a very weak suppressing effect. Neither plasmid suppressed the lesions of any of the other dna mutants.

Preferential inhibitory action of sodium cholate on an Escherichia coli strain carrying a plasmid in an integrated state

Sodium cholate was shown to be preferentially more active on Escherichia coli strains carrying an integrated plasmid, i.e., on Hfr strains, than on their parental strains with or without a plasmid in an autonomous state.

Formation of HfrH-type donor cells as a result of integrative suppression by R-F recombinant plasmids

Three recombinant plasmids, resulting from recombination between an R plasmid of the FI incompatibility group and the F of HfrH, were introduced in a temperature-sensitive dnaA mutant to isolate Hfr-type-donors. All of the temperature-insensitive clones isolated from two of the three recombinant plasmids had the same origin and transfer pattern as the parental HfrH strain.

Directed integration of an F' plasmid by integrative suppression: isolation of plaque forming lambda transducing phage for the dnaC gene

A new approach for isolation of a plaque forming lambda specialized transducing phage is described. It consists of directed transposition of an F' plasmid into the gal region of a dnaAts galE- Escherichia coli strain by integrative suppression and deletion of the chlD region in order to shorten the distance between the marker of interest on the F' and the prophage serving to prepare an LFT1 lysate. An F' danC+thr+ plasmid was used here and lambdadthr and lambdaddnaC phages were isolated. In addition, lambdapdnaC was obtained from a double lysogen for lambdaddnaC and lambda b2.

Hfr formation by I pilus-determining plasmids in Escherichia coli K-12

R483, an atypical, I pilus-determining plasmid, and also R144, a typical one, were shown to suppress the DnaA phenotype by integration into the Escherichia coli chromosome.

Chromosome-plasmid interaction in Escherichia coli K-12 carrying a thermosensitive plasmid, Rts1, in autonomous and in integrated states

An Hfr strain of Escherichia coli K-12 was obtained by integrative suppression with a thermosensitive plasmid, Rts1. The R plasmid was integrated into the chromosome between rif and thr, and transfer of the chromosome occurred counterclockwise. The thermosensitivity of host cell growth due to the dnaA mutation was markedly but not completely reduced in this integratively suppressed Hfr strain. When the dnaA mutation was removed by transducing the dnaA+ genome to this Hfr, the thermosensitivity of cell growth due to existence of Rts1 was suppressed in contrast to strains carrying it autonomously. Thermosensitivity of cell growth appeared again when the plasmid was detached from the chromosome to exist autonomously. Contrary to the effect on cell growth, the transfer of the chromosome and the plasmid itself and the ability to "restrict" T-even phages were still thermosensitive in all of these strains carrying Rts1, irrespective of its state of existence. The detached plasmid as well as the original Rts1 were segregated upon growth at 42 C. These data are discussed in relation to chromosome-plasmid interaction. One of the most important conculusions is that some plasmid genes, related to their replication, are phenotypically suppressed by the chromosome when it is integrated.