Comparative genomic and protein sequence analyses of the chemotaxis system of Azorhizobium caulinodans

Azorhizobium caulinodans ORS571 can fix nitrogen not only as a free-living organism and an associative-symbiotic bacterium by colonizing the root surface of non-leguminous plants, but also as a symbiotic bacterium by interacting with leguminous plant Sesbania rostrata. Due to its ability to grow and fix nitrogen under three conditions, A. caulinodans uses sophisticated chemotaxis signal transduction systems to transform environmental cues into corresponding behavioral responses. Chemotaxis appears crucial for the growth of A. caulinodansin complicated environment and the construction of associative relationship with the plant. However, little is known about the chemotactic pathway of A. caulinodans. Thus, our study aimed to compare the chemotaxis-like genes of A. caulinodans with those of well-studied species.

NCBI protein BLAST was used for searching sequence similarity with default parameter values against the genomes of A. caulinodans. HMMER3, based on Pfam database, was used for comparative analyses of methyl-accepting chemotaxis protein (MCP).

There was a major chemotaxis cluster in A. caulinodans and the CheR methylated MCPs independently of pentapeptide motif. There were 43 MCP homologs containing diverse signal-sensing architectures in A. caulinodans. In addition, cytoplasmic domains of these MCPs were all composed of 38 heptad repeats.

Despite the extremely high homology presented between the chemotactic system of A. caulinodans and those of well-studied species, A. caulinodans shows its own unique characteristics. The classification of these chemotactic pathways by comparative genomics enables us to better understand how A. caulinodansresponds to changes in environment via exquisite signal transductions in chemotaxis system.