IDENTIFICATION OF A PKM101 REGION WHICH CONFERS A SLOW GROWTH-RATE AND INTERFERES WITH SUSCEPTIBILITY TO QUINOLONE IN ESCHERICHIA-COLI AB1157

The effect of plasmid pKM101 on the survival of Escherichia coli AB115 7, growing in minimal medium, in the presence of a 4-quinolone DNA gyr ase inhibitor was investigated, The presence of this plasmid decreased susceptibility to the quinolone ciprofloxacin, whereas mucAB genes pr esent in a multicopy plasmid did not, The same effect of pKM101 was de tected in a recA430 mutant, confirming that it was not really related to the SOS response, In contrast, when survival assays were performed under amino acid starvation conditions, pKM101 did not confer protecti on against ciprofloxacin, All of these results indicated that the synt hesis of a product(s), different from MucAB, which was encoded by the plasmid pKM101 increased the rate of survival of the AB1157 strain in the presence of quinolone, To identify the gene(s) responsible for thi s phenotype, several plasmid derivatives carrying different portions o f pKM101 were constructed. The 2,2-kb region containing korB, traL, ko rA, and traM genes was sufficient to decrease susceptibility to quinol one, This plasmidic fragment also made the AB1157 host strain grow mor e slowly (the Slo phenotype), Moreover, the suppression of the Slo phe notype by addition of adenine to the cultures abolished the decreased susceptibility to quinolone. These results are evidence that the prote ction against quinolone conferred by this region of pKM101 in strain A B1157 is a direct consequence of the slow growth rate.