SYMBIOTIC PLASMID GENES ESSENTIAL TO THE CATABOLISM OF PROLINE BETAINE, OR STACHYDRINE, ARE ALSO REQUIRED FOR EFFICIENT NODULATION BY RHIZOBIUM-MELILOTI

Genes encoding nodulation and nitrogen fixation are grouped on the symbiotic plasmid (pSym) in a region that contains genes necessary for betaine catabolism. We used insertional mutagenesis to map genes involved in the demethylation of stachydrine (proline betaine) to a 3.5 kb region to the right of nodD2, a nodulation regulatory gene. Two insertions into this region (stc) created transcriptional fusions that were induced by stachydrine. In both of these, the lacZ reporter was oriented opposite nod box 6, which lies between nodD2 and the ate region, indicating that induction is not dependent of this nod box. All of the Stc(-) insertional mutants exhibited delayed and reduced nodulation. We take this as evidence for a functional role of stc genes in nodulation.