We have isolated a Tn5 mutant of S. meliloti GR4 showing conditional swarming. Multicellular migration in the mutant strain QS77 seems to be a cell density-dependent phenomenon induced on semisolid minimal medium in response to certain amino acids, influenced by the viscosity of the medium, and abolished by glucose. The mutation in QS77 lies within a gene encoding a homolog of the FadD protein (long chain fatty acyl CoA ligase) of several microorganisms, including the RpfB protein of the phytopathogenic Xanthomonas campestris, a putative FadD protein mediating regulation of virulence factor synthesis (Barber et al. 1997). Like an Escherichia colifadD mutant and in contrast to the S. meliloti wild type strain GR4, QS77 cannot grow using oleate as sole carbon source. Interestingly, the S. meliloti fadD mutant shows a defective symbiotic phenotype. Although QS77 is able to induce nitrogen-fixing root nodules on alfalfa plants, it exhibits a less efficient nodulation phenotype than the wild-type, together with a significant reduction in competitiveness. Furthermore, the expression of genes involved in motility {che, mofli, and flaA belonging to the flagellar regulon), and symbiosis {nodC) are significantly reduced in the fadD mutant. In trans expression of multicopy fadD restored growth ability of an E. colifadD mutant as well as growth, control of motility and symbiotic phenotype of QS77.

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