The nolL gene from Rhizobium etli determines nodulation efficiency by mediating the acetylation of the fucosyl residue in the nodulation factor

The nodulation factors (Nod factors) of Rhizobium etli and R. loti carry a 4-O-acetyl-L-fucosyl group at the reducing end, It has been claimed, based on sequence analysis, that NolL from R. loti participates in the 4-O-acetyl ation of the fucosyl residue of the Nod factors, as an acetyl-transferase ( D. B. Scott, C. A. Young, J. M. Collins-Emerson, E, A. Terzaghi, E. S. Rock man, P. A. Lewis, and C. E. Pankhurst. Mel. Plant-Microbe Interact. 9:187-1 97, 1996). Further support for this hypothesis was obtained by studying the production of Nod factors in an R. etli nolL::Km mutant. Chromatographic a nd mass spectrometry analysis of the Nod factors produced by this strain sh owed that they lack the acetyl-fucosyl substituent, having a fucosyl group instead, Acetyl-fucosylation was restored upon complementation with a wild- type nolL gene. These results indicate that the nolL gene determines 4-O-ac etylation of the fucosyl residue in Nod factors, Analysis of the predicted NolL polypeptide suggests a transmembranal location and that it belongs to the family of integral membrane transacylases (J. M, Slauch, A. A. Lee, M. J. Mahan, and J. J. Mekalanos. J, Bacteriol. 178:5901-5909, 1996). NolL fro m R. loti was also proposed to function as a transporter; our results show that NolL does not determine a differential secretion of Nod Factors from t he cell. We also performed plant assays that indicate that acetylation of t he fucose conditions efficient nodulation by R. etli of some Phaseolus vulg aris cultivars, as well as of an alternate host (Vigna umbellata).