Genes involved in lipopolysaccharide production and symbiosis are clustered on the chromosome of Rhizobium leguminosarum biovar viciae VF39

Four mutants of Rhizobium leguminosarum biovar viciae VF39 altered in lipopolysaccharide (LPS) synthesis were isolated upon random Tn5 mutagenesis. These mutants produced matt colonies on TY medium and showed autoagglutination and loss of motility. On sodium dodecyl sulfate-polyacrylamide gels, they lacked a slow-migrating carbohydrate band, corresponding to the complete LPS (LPSI). All four mutants formed small white nodules on Vicia hirsuta. These nodules were infected but showed no nitrogen-fixing activity and senesced prematurely. Three of the mutants were complemented by a wild-type cosmid to synthesis of normal LPS and induction of nitrogen-fixing nodules. By hybridization and in vivo cloning experiments, the mutations were mapped within different EcoRI fragments which could be localized on the VF39 chromosome. Cross-complementation analyses revealed that the three mutants were affected in different transcriptional units. The results indicate that a cluster of genes necessary for LPSI production and symbiotic efficiency is located within a defined region of 20 kilobases on the R. leguminosarum bv. viciae chromosome.