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The nolL Gene from Rhizobium etli Determines Nodulation Efficiency by Mediating the Acetylation of the Fucosyl Residue in the Nodulation Factor

¹Programa de Biología Molecular de Plásmidos Bacterianos, Centro de Investigación sobre Fijación de Nitrógeno, Universidad Nacional Autónoma de México, Ap. Postal 565-A, Cuernavaca, Morelos, 62271, México; ²Institut de Pharmacologie et Biologie Structurale, Spectrometrie de Masse et Signaux Biologiques, Centre National de la Recherche Scientifique, 205 Route de Narbonne 31077, Toulouse Cedex, France

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-acetylation 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. Rockman, P. A. Lewis, and C. E. Pankhurst. Mol. Plant-Microbe Interact. 9:187--197, 1996). Further support for this hypothesis was obtained by studying the production of Nod factors in an R. etli nolL::Km mutant. Chromatographic and mass spectrometry analysis of the Nod factors produced by this strain showed 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-acetylation 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. 178:5904--5909, 1996). NolL from 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 the cell. We also performed plant assays that indicate that acetylation of the fucose conditions efficient nodulation by R. etli of some Phaseolus vulgaris cultivars, as well as of an alternate host (Vigna umbellata).