IDENTIFICATION AND CHARACTERIZATION OF A NOVEL BRADYRHIZOBIUM-JAPONICUM GENE INVOLVED IN HOST-SPECIFIC NITROGEN-FIXATION

To understand the genetic mechanism of host specificity in the interac tion between rhizobia and their hosts, it is important to identify gen es that influence both early and late steps in symbiotic development. This paper focuses on the little-understood genetics of host-specific nitrogen fixation. A deletion mutant of Bradyrhizobium japonicum, stra in NAD163, was found to induce effective, nitrogen-fixing nodules on s oybean and siratro plants but produced ineffective nodules on cowpea p lants. Additional transposon and deletion mutants defined a small regi on that conferred this phenotype, and this region was sequenced to ide ntify two putative open reading frames (ORFs). Data indicate that only one of these ORFs is detectable in bacteroids. This ORF was termed hs fA, with a predicted protein product of 11 kDa. The transcriptional st art site of hsfA was determined and found to coincide with a predicted RpoN-dependent promoter. Microscopic studies of nodules induced by th e wild type and hsfA mutants on cowpea and soybean plants indicate tha t the cowpea mutant nodules are slow to develop. The data indicate tha t hsfA appears to play a crucial role in bacteroid development on cowp ea but does not appear to be essential for nitrogen fixation on the ot her hosts tested.