Molecular Mimicry of NodA Genes by an N-acetylmuramoyl-L-alanine Amidases Gene in a Nodule Forming Staphylococcus cohnii [RESEARCH NOTE

Abstract

A rare, nodule-forming, Gram-positive bacterium (Sub4) was isolated from the forage and cover crop Pueraria phaseoloides, which aligned at 99.26% sequence identity to a partial 16S rDNA sequence of Staphylococcus cohnii sp. This was the first record of a strain/species of Staphylococcus capable of independent, unassisted nodule formation in a legume host. When a nodA gene was sought by PCR using a pair of gene-specific primers synthesized using a related (Firmicutes) Paenibacillus sp. nodA gene, the reaction yielded a PCR product of similar size but a distinct identity. The resulting ~400bp PCR product coded for a likely N-acetylmuramoyl-L-alanine amidase and not an acyltransferase. It may be inferred that the N-acetylmuramoyl-L-alanine amidase gene was amplified due to sequence similarities/mimicry between 2 cosmetically unrelated loci. There was also an analogy in protein backbones between homology models of the Sub4 N-acetylmuramoyl-L-alanine amidase and a Paenibacillus nodA gene product, which also suggests that the 2 protein families may have an evolutionary relationship. When the phylogeny of the sequenced locus was inferred, a unique clade with high bootstrap support was formed by the sequence under study with an Azorhizobium nodA gene. Azorhizobia were distinct in relation to their nodule formation on stems and their ability to fix nitrogen in the free-living and the symbiotic forms. N-acetylmuramoyl-L-alanine amidases may have roles to play in remodeling peptidoglycan for LysM receptor interactions, likely without nod factors.