Construction of a recA mutant of Azospirillum lipoferum and involvement of recA in phase variation

The plant-growth promoting rhizobacterium Azospirillum lipoferum strain 4B generates in vitro a stable phase variant designated 4V I at frequencies of 10 −4 to 10 −3 per cell per generation. Variant 4V I displays pleitropic modifications, such as the loss of swimming motility and the inability to assimilate certain sugars compared to the wild type. The mechanism underlying phase variation is unknown. To determine whether RecA-mediated processes are involved in phase variation, the recA gene of A. lipoferum 4B was cloned and sequenced and a recA mutant (termed 4B recA) was constructed by allelic exchange. Strain 4B recA showed increased sensitivity to UV and MMS compared with 4B and impaired recombinase activity. The ability to generate variants in vitro was not altered; the variants from 4B recA exhibited all morphological and biochemical features characteristic of the variant generated by strain 4B. However, the frequency of variants generated by 4B recA was increased by up to 10-fold. So, in contrast with many studies showing the abolition or a large reduction of the frequency of phase variation in recA mutants, this study describes an enhancement of phase variation in the absence of a functional recA.