A NEW PHENOTYPE FOR SBCB MUTATIONS IN ESCHERICHIA-COLI - RECA-DEPENDENT INCREASE IN PLASMID-BORNE GENE-EXPRESSION

A new chromosomal mutation (cpeA), that causes increased expression of plasmid-borne genes in Escherichia coli, was identified and mapped to the sbcB locus. The effect of the mutation on plasmid transcription w as non-specific with respect to the various promoters that were studie d, but was more pronounced for an IncW low-copy-number plasmid than fo r Co1E1- or p15A-based, high-copy-number plasmids. The mutant phenotyp e was observed even in recB(+)C(+)D(+) strains, but not in recA mutant s. The increased-expression phenotype was also observed in sbcB15 but not in xonA1 (another sbcB allele) mutants, suggesting that the expres sion of this phenotype is mediated by genes of the so-called RecF path way family. Consistent with this interpretation was the observation th at the cpeA mutant phenotype was less pronounced in recF, recJ and rec O mutants. The increased-expression phenotype was also correlated with increased recovery of plasmid DNA from the cpeA/sbcB mutant strains, but there was no evidence for the occurrence of linear plasmid multime rs in these strains.