Genetics of thermotolerance in ciprofloxacin resistant mutant of Rhizobium leguminosarum bv phaseoli.

ABSTRACT

A ciprofloxacin resistant mutant (Cf(R)) of Rhizobium leguminossarum bv phaseoli USDA 2695 which nodulates common bean plants (Phaseolus vulgaris L) was isolated after nitrous acid mutagenesis. Another mutant resistant to nalidixic acid (Nal(R)) was isolated spontaneously. Both mutants showed thermotolerance as evident by their ability to grow at elevated (40 degrees C) temperature, although the wild type (USDA 2695) failed to grow at this temperature. Transformation and plasmid curing experiments suggested the gene(s) controlling thermotolerance (TrR) and resistance to nalidixic acid or ciprofloxacin were located on the main chromosome and not on the plasmids. High frequency of co-transfer of TrR-Cr(R) and Tr(R)-Nal(R) during transformation experiments indicated a close association of these gene(s). Role of DNA gyrase and supercoiling in these thermotolerant mutants has been discussed.