THE PEA LATE NODULIN GENE PSNOD6 IS HOMOLOGOUS TO THE EARLY NODULIN GENES PSENOD3 14 AND IS EXPRESSED AFTER THE LEGHEMOGLOBIN GENES/

The pea late nodulin gene PsNOD6 has been cloned and sequenced. PsNOD6 is homologous to the pea early nodulin genes PsNOD3 and PsENOD14. In situ hybridization experiments showed that, like the PsENOD3 and PsENO D14 genes, the PsNOD6 gene is only expressed in the infected cell type . The PsNOD6 gene is first expressed at the transition of the pre-fixa tion zone II into the interzone II-III (the amyloplast-rich zone prece ding the fixation zone III), whereas the early nodulin genes PsENOD3 a nd PsENOD14 are already induced in the pre-fixation zone II. Thus thes e nodulin genes encoding homologous proteins are induced at consecutiv e stages of nodule development. The expression of the late nodulin gen es encoding leghaemoglobin precedes the expression of the late nodulin gene PsNOD6. Therefore these late nodulin genes have to be regulated by different mechanisms despite the fact they are expressed in the sam e cell type. This conclusion is consistent with the fact that PsNOD6 l acks one of the conserved regions occurring in the promoters of all ot her late nodulin genes studied.