DICTYOSTELIUM-DISCOIDEUM FATTY-ACYL AMIDASE-II HAS DEACYLASE ACTIVITYON RHIZOBIUM NODULATION FACTORS

Dictyostelium discoideum (Amoebidae) secretes cell-lysing enzymes: est erases, amidases, and glycosylases, many of which degrade soil bacteri a to provide a source of nutrients, Two of these enzymes, fatty-acyl a midases FAA I and FAA II, act sequentially on the N-linked long chain acyl groups of lipid A, the lipid anchor of Gramnegative bacterial lip opolysaccharide, FAA I selectively hydrolyzes the 3-hydroxymyristoyl g roup N-linked to the proximal glucosamine residue of de-O-acylated lip id A, Substrate specificity for FAA II is less selective, but does req uire prior de-N-acylation of the proximal sugar, i.e. bis-N-acylated l ipid A is not a substrate, We have synthesized a C-14-labeled substrat e analog for FAA II and used this in a novel assay to monitor its puri fication, Inhibitory studies indicate that FAA II is not a serine prot ease, but may have a catalytic mechanism similar to metalloprotein de- N-acetylases such as LpxC, Interestingly, rhizobial Nod factor signal oligosaccharides that induce root nodules on leguminous plants have ma ny of the structural requirements for substrate recognition by FAA II, In vitro evidence indicates that Rhizobium fredii Nod factors are sel ectively de-N-acylated by purified FAA II, suggesting that the enzyme may reduce the N-2-fixing efficiency of Rhizobium-legume symbioses, In contrast, N-methylated Nod factors from transgenic R, fredii carrying the rhizobial nodS gene were resistant to FAA II, suggesting a mechan ism by which Nod factors may be protected from enzymatic de-N-acylatio n, Since FAA II and Nod factors are both secreted, and Nod factors tha t lack the N-acyl group are unable to induce nodules, dictyostelial FA A II may decrease the efficiency of symbiotic nitrogen fixation in the environment by reducing the available biologically active nodule indu cer signal.