Nucleotide sequence of the Rhizobium etli nods gene

Abstract

Bacteria of the genera Rhizobium, Bradyrhizobium and Azorhizobium can develop nitrogen-fixing nodules on leguminous plants in a host-specific way. The first stage of this interaction leads to the activation of bacterial nodulation (nod) genes by plant flavonoids. The nod genes code for Nod factors, which act as host-specific signals to trigger nodule formation. The Nod factors are oligomers of P-1,4-linked glucosamine residues, N-acylated on the terminal non-reducing sugar and N-acetylated on the other residues (DCnariC et al., 1993). The nodABC genes are involved in the biosynthesis of the core of the Nod factors. Recently, we have proposed (Vazquez et al., 1993) that Nod1 and NodJ proteins, are members of a polysaccharide secretion system that could be involved in the export of the Nod factors. Other nod Correspondence to: Dr. C. Quinto, Departamento de Biologia Molecular de Plantas, Instituto de Biotecnologia, Universidad National Autonoma de Mexico, Apdo. Postal 510- 3, Cuernavaca, Morelos CP 62271, Mexico. Tel. (52-73) 114-900; Fax (52-73) 172-388; e-mail: quinto@pBR322.ceingebi.unam.mx *On request, the authors will supply detailed experimental evidence for the conclusions reached in this Brief Note. Abbreviations: A., Azorhizobium; aa, amino acid(s); B., Bradyrhizobium; bp, base pair(s); GCG, Genetics Computer Group (Madison, WI, USA); kb, kilobase or 1000 bp; MTase, methyltransferase; nod, nodulation genes; Nod, nodulation factors; ORF, open reading frame; R., Rhizobium; SAM, S-adenosylmethionine. genes, mediate the “decoration” of the core with various substituents (N-acyl chain, sulfate, acetate, methyl and carbamoyl groups, sugars) in order to make them hostspecific. Recently, nods and nodU genes have also been identified in A. caulinodans (Geelen et al., 1993). An analysis of the deduced aa sequences of nods gene products from R. sp. NGR234, B. japonicum and A. caulinodans, revealed consensus sequences with high similarity to SAM-utilizing enzymes. In A. caulinodans, nods has been proposed to encode for a MTase involved in the modification of the Nod factor (Geelen et al., 1993). We reported a unique organization of the common nod genes in R. leguminosarum bv. phaseoli strain CE3 (Vazquez et al., 1991), reclassified as belonging to a new species, R. etli (Segovia et al., 1993). At present we are reporting that immediatly downstream from the R. etli common nodC gene (38 bp), we found a 205-aa ORF (GenBank accession No. L11750) that encodes a 22728-Da Nods displaying a strong similarity to the R. sp. strain NGR234 Nods, as well as to the deduced aa sequences of B. japonicum, R. fredii and A. caulinodans Nods that have been described as SAM-utilizing enzymes (Geelen et al., 1993). At the aa level, the ORF shows 66, 55, 54 and 31% identity with R. sp. strain NGR234, B. japonicum, R.fredii USDA257 and A. caulinodans Nods aa sequences, respectively (Fig. 1).