THE NODA PROTEINS OF RHIZOBIUM-MELILOTI AND RHIZOBIUM-TROPICI SPECIFYTHE N-ACYLATION OF NOD FACTORS BY DIFFERENT FATTY-ACIDS

Rhizobia synthesize mono-N-acylated chitooligosaccharide signals, call ed Nod factors, that are required for the specific infection and nodul ation of their legume hosts, The biosynthesis of Nod factors is under the control of nodulation (nod) genes, including the nodABC genes pres ent in all rhizobial species, The N-acyl substitution can vary between species and can play a role in host specificity, In Rhizobium melilot i, an alfalfa symbiont, the acyl chain is a C16 unsaturated or a (omeg a-1) hydroxylated fatty acid, whereas in Rhizobium tropici, a bean sym biont, it is vaccenic acid (C18:1), We constructed R. meliloti derivat ives having a nonpolar deletion of nodA, and carrying a plasmid with e ither the R. meliloti or the R. tropici nodA gene, The strain with the R. tropici nodA gene produced Nod factors acylated by vaccenic acid, instead of the C16 unsaturated or hydroxylated fatty acids characteris tic of R. meliloti Nod factors, and infected and nodulated alfalfa wit h a significant delay. These results show that NodA proteins of R. mel iloti and R. tropici specify the N-acylation of Nod factors by differe nt fatty acids, and that allelic variation of the common nodA gene can contribute to the determination of host range.