Site-specific integration of an F' lac pro factor in the region of the replication origin (oriC) of E. coli

DNA replication termination in Escherichia coli parB (a dnaG allele), parA, and gyrB mutants affected in DNA distribution

We investigated the Escherichia coli mutants carrying the parB, parA, and gyrB mutations, all of which display faulty chromosome partitioning at the nonpermissive temperature, to see whether their phenotype reflected a defect in the termination of DNA replication. In the parB strain DNA synthesis slowed down at 42 degrees C and the SOS response was induced, whereas in the parA strain DNA synthesis continued normally for 120 min and there was no SOS induction. To see whether replication forks accumulated in the vicinity of terC at the nonpermissive temperature, the mutants were incubated for 60 min at 42 degrees C and then returned to low temperature and pulse-labeled with [3H]thymidine. In all cases the restriction pattern of the labeled DNA was incompatible with that of the terC region, suggesting that replication termination was normal. In the parA mutant no DNA sequences were preferentially labeled, whereas in the parB and gyrB strains there was specific labeling of sequences whose restriction pattern resembled that of oriC. In the case of parB this was confirmed by DNA-DNA hybridization with appropriate probes. This test further revealed that the parB mutant over initiates at oriC after the return to the permissive temperature. Like dna(Ts) strains, the parB mutant formed filaments at 42 degrees C in the absence of SOS-associated division inhibition, accompanied by the appearance of anucleate cells of nearly normal size (28% of the population after 3 h), as revealed by autoradiography. The DNA in the filaments was either centrally located or distributed throughout. The parB mutation lies at 67 min, and the ParB- phenotype is corrected by a cloned dnaG gene or by a plasmid primase, strongly suggesting that parB is an allele of dnaG, the structural gene of the E. coli primase. It is thus likely that the parB mutant possesses an altered primase which does not affect replication termination but causes a partial defect in replication initiation and elongation and in chromosome distribution.

Suppressor mutations (rin) that specifically suppress the recA+ dependence of stable DNA replication in Escherichia coliK-12

The sdrA102 mutation confers upon cells the ability to replicate DNA in the absence of protein synthesis. This mutation was combined with the recA200 mutation, which renders the recA protein thermolabile, and had little effect on normal replication. However, the sdrA102 recA200 double mutant exhibited temperature-sensitive stable DNA replication: it replicated DNA continuously in the presence of chloramphenicol at 30 degrees C, whereas at 42 degrees C DNA replication ceased after the DNA content increased only 40-45%. Suppressor mutants (rin; recA-independent) capable of stable DNA replication at 42 degrees C were isolated from the double mutant. The suppressor mutant retained all other recA- characteristics, i.e., deficient general recombination, severe UV-sensitivity, and incapability of prophage induction in lysogens. This indicates that the rin mutation specifically suppresses the recA+ dependency of stable DNA replication. It is suggested that the recA+ protein stabilizes a specific structure, similar to an intermediate in recombination, which may function in the initiation of stable DNA replication.

Discontinuity of homology of Escherichia coli and Salmonella typhimurium DNA in the lac region

Partial homology of Salmonella typhimurium DNA to Escherichia coli DNA was demonstrated by Southern hybridization blots to exist on either side of the lac operon of E. coli but no homology was detected between S. typhimurium DNA and about 12 kb of E. coli DNA including the lac genes as well as about 5 kb of E. coli DNA between lac and proC. Thus portions of DNA seem to have been either added to the E.coli genome or deleted from the S. typhimurium genome since their divergence from a common ancestor. Although an IS1 element was located near the lac operon of E. coli, the insertional element was shown not to be near any of the junctures of discontinuity of E. coli--S. typhimurium homology near lac.

Trimethoprim-produced F-specific insertion mutations in Escherichia coli K-12

Besides producing thymine-requiring mutants (thy), trimethoprim (TMP) cured the mini-ColE1 replicon pML21 at an appreciable frequency. The cured Escherichia coli K-12 cells behaved like polA mutants by failing to support the stable maintenance of the ColE1 plasmid. The mini-F replicon pSC138, which was lacking all three insertion sequences (IS3, gammadelta, and IS2) normally used for F-specific integration and excision, was not cured by TMP. Instead, it integrated into specific regions of the E. coli chromosome and thus caused auxotrophic mutations in operons which were always localized on either side of oriC (origin of chromosomal replication). The incompatibility and replication functions of the integrated plasmid in auxotrophs were retained, and the plasmid DNAs recovered from spontaneously occurring revertants did not show any alterations in their contour lengths as determined by electron microscopy. The F replicon (fragment 5) contained in plasmid pSC138 carried two origins of replication, the primary origin, oriV(1) at 42.6F and the secondary origin, oriV(2), at 44.1F. Another mini-F plasmid pMF21, deleted of the primary origin of replication (oriV(1)), was still capable of autonomous replication but failed to integrate onto the chromosome after TMP treatment. Furthermore, the composite plasmid pRS5, which normally uses only the replication origin and functions of the pSC101 component, was also insensitive to TMP. On the basis of these results, we propose a new scheme of F integration via the functional oriV(1) and suggest the involvement of a similar mechanism in the formation of Hfr strains by integrative suppression.

Localization of phoE, the structural gene for outer membrane protein e in Escherichia coli K-12

To localize the structural gene for outer membrane protein e, mutants resistant to the protein e-specific phage TC45 were isolated and characterized. Three classes of TC45-resistant strains were found: (i) mutants in phoB, a regulatory gene for protein e, (ii) mutants with an altered lipopolysaccharide, and (iii) mutants unaltered in the regulation of the pho regulon and producing an apparently normal lipopolysaccharide. Mutations in the latter class of mutants are probably altered in the structural gene for protein e and are cotransducible with proA,B at min 6 on the chromosomal map. As class (iii) includes mutants with an electrophoretically altered protein e, in both an nmpA and an nmpB background, we conclude that the structural gene for protein e, designated as phoE, is localized at min 6 on the chromosomal map, the gene order being phoE proA argF.