The bio operon on the acquired symbiosis island of Mesorhizobium sp. strain R7A includes a novel gene involved in pimeloyl-CoA synthesis.

The symbiosis island of Mesorhizobium sp. strain R7A is a 500 kb chromosomal genetic element that upon transfer converts nonsymbiotic mesorhizobia to symbionts able to nodulate and fix nitrogen with Lotus corniculatus. Four genomic species of nonsymbiotic mesorhizobia have been isolated. All were auxotrophic for thiamin and biotin and three were auxotrophic for nicotinate, whereas derivatives of the strains containing the symbiosis island were prototrophic for all three vitamins. In this work, a 13.2 kb region of the island that converts the nonsymbionts to nicotinate and biotin prototrophy was characterized. The region contained orthologues of the Escherichia coli bioBFD and A genes arranged in an operon with a novel gene, bioZ, a nadABC operon, the nitrogen-fixation regulatory gene nifA, and a homologue of the pantothenate biosynthesis gene panD. The bioZ gene product was similar to beta-ketoacyl-acyl carrier protein synthase III (FabH). bioZ::Tn5 mutants grew poorly in the absence of biotin and the bioZ gene complemented an E. coli bioH mutant, suggesting that its product is involved in the synthesis of pimeloyl-COA: The bio operon was not required for symbiosis, as only mutants in the nifA gene were impaired in symbiosis, and a bioA::Tn5 mutant was not impaired in rhizosphere colonization. The rationale for the vitamin biosynthetic loci being located on an acquired genetic element that is absent from nonsymbiotic mesorhizobia remains to be determined.