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 sym biotic plasmid (pSym) in a region that contains genes necessary for be taine catabolism. We used insertional mutagenesis to map genes involve d in the demethylation of stachydrine (proline betaine) to a 3.5 kb re gion to the right of nodD2, a nodulation regulatory gene. Two insertio ns into this region (stc) created transcriptional fusions that were in duced by stachydrine. In both of these, the lacZ reporter was oriented opposite nod box 6, which lies between nodD2 and the ate region, indi cating that induction is not dependent of this nod box. All of the Stc (-) insertional mutants exhibited delayed and reduced nodulation. We t ake this as evidence for a functional role of stc genes in nodulation.