Molecular characterization and regulation of the rhizosphere-expressed genes rhiABCR that can influence nodulation by Rhizobium leguminosarum biovar viciae

• Published in: Journal of Bacteriology Vol. 174; no. 12; pp. 4026 - 4035
• Main Authors: Cubo, M.T. (Universidad de Sevilla, Seville, Spain), Economou, A, Murphy, G, Johnston, A.W.B, Downie, J.A
• Format: Journal Article
• Language: English
• Published: Washington, DC American Society for Microbiology 01.06.1992
• Subjects: Amino Acid Sequence; Bacteria; Bacterial Proteins - chemistry; Bacterial Proteins - genetics; Bacteriology; Base Sequence; Biological and medical sciences; DNA-Binding Proteins; Flavonoids - pharmacology; Flowers & plants; FORMATION DE NODOSITES; Fundamental and applied biological sciences. Psychology; GENE; Gene Expression Regulation, Bacterial - drug effects; Gene Expression Regulation, Bacterial - genetics; GENES; GENETICA; Genetics; GENETIQUE; Hesperidin; Medical research; Microbiology; Molecular Sequence Data; NODULACION; NUCLEOTIDE; NUCLEOTIDOS; Operon - genetics; PLASMIDE; PLASMIDIOS; Plasmids - genetics; Recombinant Fusion Proteins - genetics; Recombinant Fusion Proteins - metabolism; RHIZOBIUM LEGUMINOSARUM; Rhizobium leguminosarum - genetics; Sequence Alignment; Trans-Activators; VICIA HIRSUTA; Symbionte; Nucleotide sequence; Gene product; Sequence alignment; Nodulation; Vegetal microorganism relation; Homology; Rhizobium leguminosarum; Regulation(control); Leguminosae; Gene; Dicotyledones; Angiospermae; Bacteria; Spermatophyta; Rhizobiaceae; Aminoacid sequence; Vicia hirsuta
• ISSN: 0021-9193; 1098-5530; 1067-8832
• DOI: 10.1128/jb.174.12.4026-4035.1992

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 (rhiABC) transcribed convergently toward the fourth gene, rhiR. rhiABC are under the positive control of RhiR, the expression of which 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