The Rhizobium leguminosarum bv. viciae NodO protein compensates for the exported signal made by the host-specific nodulation genes

Abstract

The nodulation of peas and vetches by R. leguminosarum biovar viciae requires the expression of several bacterial nodulation (nod) genes (1). One of these (nodD) is expressed constitutively and is involved in the positive regulation of the other nod genes (2). Flavonoids such as naringenin and eriodictyol are secreted from pea roots (3,4) and appear to be recognized by the NodD protein (5,6) which is then involved in the activation of transcription of the other nod genes. This regulation is mediated by the NodD protein binding to the promoters of the other nod genes (7) at a highly conserved DNA sequence (nod-box). In R. meliloti there also appears to be a repressor of nod gene expression (8) but we have not found such a repressor in b.v. viciae. However, we have identified mutations that lower the level of induction of nod gene expression. Recombinant plasmids which suppress these mutations have been identified and DNA hybridization using these clones has shown that they are from outside the nod gene region.