A small gene family of broad bean codes for late nodulins containing conserved cysteine clusters

Five transcripts encoding different members of a nodulin family with conserved cysteine clusters (Cys-X-4-Asp-Cys and Cys-X-4-Cys) were identified in broad bean root nodules. They displayed homologies to the early nodulins PsENOD3 and PsENOD14 and the late nodulin PsNOD6 from pea. In addition to the occurence of putative secretory signal peptides, the spatial distribution of the cysteine residues was comparable in both the broad bean and the pea nodulins. Based on tissue print hybridizations, we found that the corresponding broad bean genes VfNOD-CCP1, VfNOD-CCP3 and VfNOD-CCP5 were expressed in the interzone II-III and the nitrogen fixing zone III of mature nodules whereas the gene VfNOD-CCP4 was first induced in the prefixing zone II. A strong expression of the VfNOD-CCP2 gene only could be detected the interzone II-III region. Sequence analysis of a genomic VfNOD-CCP1 clone isolated revealed the presence of one intron seperating a first exon encoding the signal peptide from a second exon encoding the cysteine cluster domain of this nodulin. Apart from the multiple presence of the common nodulin motifs AAAGAT and CTCTT on both DNA strands of the putative VfNOD-CCP1 promoter region a sequence element resembling the organ specific element of the soybean lbc3 gene promoter was identified. (C) 2000 Published by Elsevier Science Ireland Ltd. All rights reserved.