CONICET | Buscador de Institutos y Recursos Humanos

Bradyrhizobium diazoefficiens, the soybean N-fixingsymbiont, possesses a dual flagellar system comprising a constitutivesubpolar flagellum and inducible lateral flagella. Here, we analyzed the genomic organization andbiosynthetic regulation of the lateral-flagellar genes. We found that those genes are located in asingle genomic cluster, organized in two monocistronic transcriptional units and threeoperons, one possibly containing an internal transcription start site. Among themonocistronic units is blr, homologous to the Class-IB master regulators of flagella synthesis in Brucella melitensis and Ensifer meliloti, and required for the expression of all the lateral-flagellar genesexcept lafA, which locusencodes a single lateral flagellin. We therefore named blr lafR (lateral-flagellarregulator). Despite its similarity to two-component responseregulators and its possession of a phosphorylable Asp residue, lafR behaved as an orphan response regulator by not requiringphosphorylation at this site. Among the genes induced by lafR is flbTL, a Class-III regulator. We observed differentrequirements for FlbTL in the synthesis of each flagellin subunit.While accumulation of lafA, but not lafA, transcripts required FlbTL; the production of bothflagellin polypeptides requiredFlbTL . Moreover, theregulation cascade of this lateral-flagellar regulon appeared not as strictly ordered asthose found in other bacterial species.