A group of four rhi (rhizosphere-expressed) genes from the symbiotic plasmid of Rhizobium leguminosarum biovar viciae has been characterized. Although mutation of the rhi genes does not normally affect nodulation, in the absence of the closely linked nodulation genes nodFEL, mutations in the rhi genes can influence the nodulation of the vetch Vicia hirsuta. The DNA sequence of the rhi gene region reveals four large open reading frames, three of them constituting an operon (rhiL4BC) transcribed convergently toward the fourth gene, rhiR. rhL4BC are under the positive control of RhiR, the expression ofwhich is repressed by flavonoids that normally induce nod gene expression. This repression, which requires the nodD gene product (the transcriptional activator of nod gene expression), may be due to a cis effect caused by a high level of NodD-dependent expression from the adjacent nodO promoter, which is transcribed divergently from rhiR. RhiR shows significant similarities to a subfamily of transcriptional regulators that includes the LuxR and UvrC-28K proteins. RhiA shows limited homology to a short domain of the lactose permease, LacY, close to a region thought to be involved in substrate binding. No strong homologies were found for the other rhi gene products. It appears that RhiA and RhiB are cytoplasmic, whereas RhiC is a periplasmic protein, since it has a typical N-terminal transit sequence and a rhiC-phoA protein fusion expresses alkaline phosphatase activity. The biochemical role of the rhi genes has not been established, but it appears that they may play a role in the plant-microbe interaction, possibly by allowing the bacteria to metabolize a plant-made metabolite.There are many bacterial genes involved in the interaction between rhizobia and their legume hosts. In Rhizobium spp. many of these genes are present on large symbiotic plasmids. Initially, these genes were identified by isolating mutants unable to fix nitrogen or form nodules, but several nodulation (nod) genes in which mutations have little or no effect on nodulation have now been identified. Currently, over 30 different nod genes have been identified among a wide variety of rhizobia (32), and in general they are under the control of positively acting transcriptional regulators encoded by nodD genes. Several of the nod gene products are involved in the biosynthesis of low-molecular-weight signalling molecules that are specifically recognized by legumes. It is now clear that different rhizobia make different but related signalling molecules which are substituted glycolipids consisting of acylated oligoglucosamine signal molecules (43,51). The biosynthesis of these nodulation factors involves most of the nod gene products; different substitutions of the glycolipid, such as the presence of sulfate or the type of acyl group, are mediated by nod gene products and determine host specificity in the interaction between the bacterium and legume (43,51).In addition to the various nod genes and genes involved in nitrogen fixation, several other symbiotic-...

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