| dc.description.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). | |
