Involvement of recA and recF in the induced precise excision of Tn10 in Escherichia coli

Mistranslation and genetic variability: the effect of streptomycin

Streptomycin is an aminoglycoside antibiotic that acts at the level of protein synthesis. Exposure to sublethal concentrations of this antibiotic increased significantly the number of Arg+ mutants derived from an Escherichia coli argE3 (ochre) rpsL31 (streptomycin-resistant) strain. The vast majority of these mutants appeared on selective minimal medium plates with streptomycin (200 micro g/ml) during stationary phase, after 6-10 days incubation at 37 degrees C. Derivative mutD5 or mutL or mutS mutants, carrying a faulty epsilon subunit of DNA polymerase or a defective mismatch DNA-repair protein, respectively, also showed higher numbers of Arg+ mutants on selective medium with streptomycin than on medium without streptomycin. Interestingly, with these DNA-repair mutants about 50% of the Arg+ mutants generated in the presence of streptomycin appeared during the first 5 days of incubation. These observations suggest that the activities of these fidelity-repair proteins prevent in the parental strain the early appearance of the supernumerary Arg+ mutants on the selective medium with streptomycin. The appearance of Arg+ mutants on the plates with streptomycin was not significantly altered by recA, rpoS or dps mutations. A high percentage of the Arg+ mutants arising in the presence of streptomycin were streptomycin-dependent for growth without arginine (Arg+ St-D). These types of mutants displayed a Ram (for ribosomal ambiguity) phenotype, manifested by increased misreading, assayed by in vitro and in vivo experiments and by leakiness on several selective minimal media. Genetic data indicated that these mutants carry a mutation located at about 74 min of the E.coli map that relieves the high translational fidelity conferred by the rpsL mutation. These studies suggest that the growth-limiting conditions of the assay system used, as well as the presence of streptomycin, which causes an increased production of altered proteins, favours the appearance and growth of compensatory Arg+ mutants.

On the participation of RecB in the induction of mini-Tn10 precise excision in a dnaB thermosensitive mutant

Precise excision of transposons Tn10 and mini-Tn10 is increased in the dnaB252 thermosensitive mutant of Escherichia coli K12, at the permissive temperature. DNA repair proteins like Pol II, RecF, Ruv and RecA were found to participate, to different extents, in this induced excision event. In this work we report that DNA repair-recombination protein RecBCD has a predominant role in this deletion process. The role of this and other repair proteins in DNA replication of the dnaB mutant in relation to the excision of the transposon is analyzed.

Participation of DNA polymerase II in the increased precise excision of Tn10

In this work the involvement of polymerase II (Pol II) in the precise excision of Tn10 stimulated by a dnaB252 thermosensitive (Ts) mutant at the permissive temperature, by a uvrD mutant, or by mitomycin C (MMC) or ultraviolet (UV) light treatment, was investigated. A deltapolB::kan mutant showed a significant decrease in the excision of Tn10 induced by the dnaB mutation, or by MMC or UV treatment, indicating the participation of Pol II in this type of deletion process. However, no effect of Pol II was evidenced in the excision of Tn10 stimulated by the uvrD mutation. The effect of the polB mutation on Tn10 precise excision induced by all these treatments was compared to that of mutations in repair-recombination genes recF and recA. The results reveal that the degree of participation of these genes varies depending on the agent that stimulates the deletion event.

Magnetic field exposure enhances DNA repair through the induction of DnaK/J synthesis

In contrast to the common impression that exposure to a magnetic field of low frequency causes mutations to organisms, we have demonstrated that a magnetic field can actually enhance the efficiency of DNA repair. Using Escherichia coli strain XL-1 Blue as the host and plasmid pUC8 that had been mutagenized by hydroxylamine as the vector for assessment, we found that bacterial transformants that had been exposed to a magnetic field of 50 Hz gave lower percentages of white colonies as compared to transformants that had not been exposed to the magnetic field. This result was indicative that the efficiency of DNA repair had been improved. The improvement was found to be mediated by the induced overproduction of heat shock proteins DnaK/J (Hsp70/40).

lon incompatibility associated with mutations causing SOS induction: null uvrD alleles induce an SOS response in Escherichia coli

The uvrD gene in Escherichia coli encodes a 720-amino-acid 3'-5' DNA helicase which, although nonessential for viability, is required for methyl-directed mismatch repair and nucleotide excision repair and furthermore is believed to participate in recombination and DNA replication. We have shown in this study that null mutations in uvrD are incompatible with lon, the incompatibility being a consequence of the chronic induction of SOS in uvrD strains and the resultant accumulation of the cell septation inhibitor SulA (which is a normal target for degradation by Lon protease). uvrD-lon incompatibility was suppressed by sulA, lexA3(Ind(-)), or recA (Def) mutations. Other mutations, such as priA, dam, polA, and dnaQ (mutD) mutations, which lead to persistent SOS induction, were also lon incompatible. SOS induction was not observed in uvrC and mutH (or mutS) mutants defective, respectively, in excision repair and mismatch repair. Nor was uvrD-mediated SOS induction abolished by mutations in genes that affect mismatch repair (mutH), excision repair (uvrC), or recombination (recB and recF). These data suggest that SOS induction in uvrD mutants is not a consequence of defects in these three pathways. We propose that the UvrD helicase participates in DNA replication to unwind secondary structures on the lagging strand immediately behind the progressing replication fork, and that it is the absence of this function which contributes to SOS induction in uvrD strains.

Enhanced Tn10 and mini-Tn10 precise excision in DNA replication mutants of Escherichia coli K12

The precise excision of transposon Tn10 and a mini-Tn10 derivative, inserted in the gal or lac operons, was studied in dnaB252 and dnaE486 temperature-sensitive mutants of Escherichia coli. dnaB codes for a DNA replication helicase and dnaE for the alpha subunit of DNA polymerase III. Mutations in these genes were found to enhance, at the permissive temperature, the precise excision of both genetic elements. The increase factor was much more pronounced for the dnaB252 mutant with the transposons inserted in gal. The stimulated excision was only partially affected by a recA null mutation but was significantly reduced by introduction of recF null or ruvA mutations. A model involving template switching of the polymerase between the direct repeats flanking the transposons, on the same strand or between sister strands, could account for the observed results.

RecBC and RecF recombination pathways and the induced precise excision of Tn10 in Escherichia coli

Mitomycin C (MMC) treatment or mutations in uvrD enhance the frequency of Tn10 precise excision. We have shown previously that several repair-recombination genes, such as recA, ruv and recF are involved in the induced excision process. In this study, we find that other genes belonging to the RecBC and RecF sexual recombination pathways also participate in this process since mutations in recB, sbcB or recO diminish, though to different degrees, the frequency of Tn10 precise excision induced by MMC treatment or by uvrD mutants. Pairwise combinations of some of these mutations were also tested for Tn10 induced precise excision; most of these double mutants showed additive effects in reducing the frequency of the excision process. The results of these studies suggest that recombinational-repair genes, particularly recF, sbcB and recO have different roles in the induced excision of Tn10 than in recombinational mating.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.