Transcriptional Control of the Lateral-Flagellar Genes of Bradyrhizobium diazoefficiens.

, a soybean N-fixing symbiont, possesses a dual flagellar system comprising a constitutive subpolar flagellum and inducible lateral flagella. Here, we analyzed the genomic organization and biosynthetic regulation of the lateral-flagellar genes. We found that these genes are located in a single genomic cluster, organized in two monocistronic transcriptional units and three operons, one possibly containing an internal transcription start site. Among the monocistronic units is blr6846, homologous to the class IB master regulators of flagellum synthesis in and and required for the expression of all the lateral-flagellar genes except , whose locus encodes a single lateral flagellin. We therefore named blr6846 (teral-lagellar egulator). Despite its similarity to two-component response regulators and its possession of a phosphorylatable Asp residue, behaved as an orphan response regulator by not requiring phosphorylation at this site. Among the genes induced by is , a class III regulator. We observed different requirements for FlbT in the synthesis of each flagellin subunit. Although the accumulation of , but not , transcripts required FlbT, the production of both flagellin polypeptides required FlbT Moreover, the regulation cascade of this lateral-flagellar regulon appeared to be not as strictly ordered as those found in other bacterial species. Bacterial motility seems essential for the free-living style in the environment, and therefore these microorganisms allocate a great deal of their energetic resources to the biosynthesis and functioning of flagella. Despite energetic costs, some bacterial species possess dual flagellar systems, one of which is a primary system normally polar or subpolar, and the other is a secondary, lateral system that is produced only under special circumstances. , an N-fixing symbiont of soybean plants, possesses dual flagellar systems, including the lateral system that contributes to swimming in wet soil and competition for nodulation and is expressed under high energy availability, as well as under requirement for high torque by the flagella. The structural organization and transcriptional regulation of the 41 genes that comprise this secondary flagellar system seem adapted to adjust bacterial energy expenditures for motility to the soil's environmental dynamics.