Transcriptional control of the lateral-flagellar genes of Bradyrhizobium diazoefficiens

MONGIARDINI, ELÍAS J.; QUELAS, J. IGNACIO; DARDIS, CAROLINA; ALTHABEGOITI, M. JULIA; LODEIRO, ANÍBAL R.

JOURNAL OF BACTERIOLOGY

AMER SOC MICROBIOLOGY

Año: 2017

0021-9193

Bradyrhizobium diazoefficiens, the soybean N2-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 those 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 flagella synthesis in Brucella melitensis and Ensifer meliloti, and required for the expression of all the lateral-flagellar genes except lafA2, which locus encodes a single lateral flagellin. We therefore named blr6846 lafR (lateral-flagellar regulator). Despite its similarity to two-component response regulators and its possession of a phosphorylable Asp residue, lafR behaved as an orphan response regulator by not requiring phosphorylation at this site. Among the genes induced by lafR is flbTL, a Class-III regulator. We observed different requirements for FlbTL in the synthesis of each flagellin subunit. While accumulation of lafA1, but not lafA2, transcripts required FlbTL; the production of both flagellin polypeptides required FlbTL. Moreover, the regulation cascade of this lateral-flagellar regulon appeared not as strictly ordered as those found in other bacterial species.