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The NodA proteins of Rhizobium meliloti and Rhizobium tropici specify the N‐acylation of Nod factors by different fatty acids

Rhizobia synthesize mono‐N‐acylated chitooligosaccharide signals, called Nod factors, that are required for the specific infection and nodulation of their legume hosts. The biosynthesis of Nod factors is under the control of nodulation (nod) genes, including the nodABC genes present in all rhizobial species. The N‐acyl substitution can vary between species and can play a role in host specificity. In Rhizobium meliloti, an alfalfa symbiont, the acyl chain is a C16 unsaturated or a (ω‐1) hydroxylated fatty acid, whereas in Rhizobium tropici, a bean symbiont, it is vaccenic acid (C18:1). We constructed R. meliloti derivatives having a non‐polar deletion of nodA, and carrying a plasmid with either 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 characteristic of R. meliloti Nod factors, and infected and nodulated alfalfa with a significant delay. These results show that NodA proteins of R. meliloti and R. tropici specify the N‐acylation of Nod factors by different fatty acids, and that allelic variation of the common nodA gene can contribute to the determination of host range.