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

The grpD55 locus of Escherichia coli appears to be an allele of dnaB

Attempts to characterize the grpD55 mutation of Escherichia coli have led us to conclude that the gene had been assigned an incorrect map position. The mutation was found to cotransduce with malF3089:: Tn10 (at approximately 91.5 min) and a dnaB-expressing plasmid was able to complement fully the grpD55 defect in lambda replication. These studies strongly suggest that grpD55 is an allele of dnaB and is localized near 92 min on the E. coli linkage map.

Transient suppression of F-plasmid incompatibility in a strain of Escherichia coli

The incompatibility between F plasmids is transiently suppressed in Escherichia coli strain CSH54. As a result this strain is able to maintain two F' factors or an F' factor and a mini-F plasmid for considerably longer periods than normal strains. When selective pressure for two markers carried by two separate F's (or an F' and mini-F) is imposed on normal strains, the two plasmids tend to form a cointegrate structure which can be detected genetically by the joint transfer of both the markers upon mating. This does not happen in CSH54; instead, the two plasmids are maintained and transferred independently. Physical evidence for the maintenance of an F' and a mini-F plasmid is provided by agarose gel electrophoresis.

dnaB analog function associated with plasmid R100drd-1

Plasmid R100 and a number of its derivatives were able to suppress the temperature sensitivity of strains carrying different alleles of the dnaB gene of Escherichia coli K-12. R100drd-l and pAR132 were able to rescue a strain carrying the dnaB266(Am) mutation in the absence of any known amber suppressors. This was taken as evidence for the existence of an R100drd-l dnaB analog function. The R100drd-l dnaB analog was different from those of bacteriophages P1 and P7 in that it was able to support the growth of bacteriophage lambda in a dnaB266(Am) background. The dnaB analog was also shown to be thermosensitive. The structural gene for this protein lies within the EcoRI fragment D of R100drd-l.

Antibiotic resistance and R-plasmids in food chain Salmonella: evidence of plasmid relatedness

A large number of strains (1,783) belonging to 15 Salmonella serovars isolated, in Canada, from the three major links of the human food chain were screened for multiple antibiotic resistance and the presence of R-plasmids. Multiresistant strains occurred among animal feed, livestock, and human isolates at frequencies of 4, 22, and 14%, respectively. Conjugation analysis revealed that 58% of the isolates from feeds, 87% of those from livestock, and 89% of the human strains carried all or part of their resistance determinants extrachromosomally on R-plasmids. Conjugative plasmids representing nine different incompatibility groups were detected, with the Inc I alpha group being predominant. Within the limits of the parameters measured, certain of these plasmids show a degree of relatedness suggestive of a common ancestry.

Surface exclusion between F' plasmids in strains of Escherichia coli K-12 carrying a dnaB mutation, in the presence or absence of bacteriophage genomes providing a dnaB analog function

In a set of isogenic strains, three out of four different dnaB(ts) mutations reduced surface exclusion between F' plasmids. In further studies with a strain carrying one of these mutations (dnaB43), surface exclusion remained reduced in the presence of a recombinant plasmid carrying only the region of F that encodes the surface exclusion proteins TraSp and TraTp. The dnaB analog specified by bacteriophage P1 but not that specified by P7 increased the surface excluding ability of the strain carrying dnaB43 to the same level as the isogenic dnaB+ strain. The possible role of the dnaB gene in surface exclusion is discussed.

A dnaB analog function specified by bacteriophage P7 and its comparison to the similar function specified by bacteriophage P1

Evidence is presented that bacteriophage P7 specifies an analog of the E. coli DNA replication protein, dnaB. As in the related bacteriophage P1 (D'Ari et al., 1975; Ogawa, 1975), in lysogens of P7, the production of the analog protein is repressed and constitutive mutants could be isolated. Such constitutive mutants could suppress efficiently the thermosensitivity of several dnaB(ts) mutations and also rescue a strain carrying a dnaB amber mutation. While neither P7 nor the mutant P1bacban (defective in the structural gene ban) could suppress dnaB(ts) mutations efficiently, recombinants between these two phages could do so, indicating the presence of a functional dnaB analog gene (called sdb) on P7. In a dnaB amber strain suppressed by the presence of the constitutive mutant P7csb, bacteriophage lambda failed to replicate which is a further similarity between P7 and P1. P7csb mutants or P7-P1bacban recombinants were found to be less thermoresistant than P1bac1 suggesting that the P7-specified dnaB analog protein or its production is relatively less tolerant of temperatures above 37 degrees C.

In vitro system for the replication of the mini R1 factor Rsc11

The in vitro synthesis of the mini R1-factor, Rsc11, was achieved using a soluble Escherichia coli cell-extract system. Triton X-100 lysates of the K12 strain 1101 (Rsc11) fractionated by Sephadex G25 chromatography supported the incorporation of labeled deoxyribonucleotides into covalently-closed circular (30S) and open-circular (25S) plasmid DNA as well as other molecules of various sizes. DNA synthesis required the presence of all four ribonucleotides and was rifampicin sensitive. Pulse-chase experiments indicated that this reaction is discontinuous. A dependence on ATP and sensitivity to nalidixic acid suggested this system capable of the replicative synthesis of Rsc11 DNA. Density-shift analysis confirmed this. In addition to hybrid, fully-heavy plasmid supercoils were synthesized indicating that more than one round of replication was completed. Approximately one-third of the molecules available to the system participate in this reaction.

Analogs of the dnaB gene of Escherichia coli K-12 associated with conjugative R plasmids

The dnaB266(Am) mutation in Escherichia coli K-12 is an amber mutation such that strains carrying this mutation are not viable in a sup+ strain. With five different R plasmids, it has been possible to construct viable R+ derivatives of this amber mutant and show that the plasmids themselves do not carry amber suppressors. This is interpreted as evidence for the presence of dnaB analog genes associated with these plasmids. Plasmid-positive strains carrying these genes often showed some degree of cryosensitivity of DNA synthesis and colony-forming ability. These observations indicate that the presence of dnaB analog genes in association with R plasmids must be relevant to the plasmid state or to some aspect of conjugative ability.

Expression and stability of Escherichia coli F-prime factors in Proteus mirabilis

The expression and stability of Escherichia coli F-primes in Proteus mirabilis is examined. It is possible to consecutively introduce, and stably maintain, the DNA of several E. coli F-primes in P. mirabilis in the absence of selective pressure for all or some of the plasmids. Additionally, we can recover more than one F-prime from certain P. mirabilis recipient strains which carry DNA derived from several independent matings with E. coli F-prime donors.

Role of dnaB43 in F'-plasmid incompatibility

In order to perform complementation tests with mutations of DNA replication of F'-plasmids incompatibility must overcome. We report out in ability to duplicate the results presented by Palchoudhury and Iyer (1971) and Bezanson and Iyer (1975) who have claimed to demonstrate the autonomous replication two incompatible F'-plasmids in a strain carrying the temperature sensitive dna B43 allele. In addition, we describe experiments designed to measured complementation using transient heterozygotes and compatible plasmids. Assessment of our data and those of others in the light of a recent report by Uhlin and Nordström (1975) suggests that new approaches will have to be developed for the successful employment of complementation analysis in F-plasmid genetics.

Electron microscope study of a plasmid chimera containing the replication region of the Escherichia coli F plasmid

pML31, a plasmid chimera constructed to contain the replication genes of an Flac plasmid, has been studied by electron microscope methods. Heteroduplex analysis shows that the only F sequence present in pML31 is that with corrdinates 40.3-49.3F. This region has previously been identified as essential for plasmid maintenance. The sequence of pML31, which was derived originally from R6-5, carries the km gene(s) and an inverted duplication of a 1.0-kilobase sequence. On the basis of length measurements, the repeated sequence is different from IS1, IS2, IS3, and an inverted repeat associated with the km gene(s) of plasmid JR67.

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.