Synthesis of an opine-like compound, a rhizopine, in alfalfa nodules is symbiotically regulated.

We show that the promoter of the mos locus, which encodes genes required for the synthesis of a nodule-specific, opine-like compound, a rhizopine, in alfalfa nodules is regulated by the symbiotic nitrogen-fixation regulatory gene nifA. The 5'-regulatory region and amino-terminal end of the first open reading frame of the mos locus are highly homologous to the 5'-regulatory region and amino-terminal portion of the Rhizobium meliloti nifH gene. The coordinate regulation of mos and nif genes suggests that the mos locus plays a symbiotic role. We propose that the rhizopine enhances the survival of the bacterial partner in the symbiosis.