Moderate levels of urease activity (ca. 300 mU mg-1) were detected in Rhizobium leguminosarum bv. viciae UPM791 vegetative cells. This activity did not require urea for induction and was partially repressed by the addition of ammonium into the medium. Lower levels of urease activity (ca. 100 mU mg-1) were detected also in pea bacteroids. A DNA region of ca. 9 kb containing the urease structural genes (ureA, ureB and ureC), accessory genes (ureD, ureE, ureF, and ureG), and five additional ORFs (orf83, orf135, orf207, orf223, and orf287) encoding proteins of unknown function was sequenced. Three of these ORFs (orf83, orf135 and orf207) have a homologous counterpart in a gene cluster from Sinorhizobium meliloti, reported to be involved in urease and hydrogenase activities. R. leguminosarum mutant strains carrying Tn5 insertions within this region exhibited a urease-negative phenotype, but induced wild-type levels of hydrogenase and nitrogenase activities in bacteroids. orf287 encodes a potential transmembrane protein with a C-terminal GGDEF domain. A mutant affected in orf287 exhibited normal levels of urease activity in culture cells. Experiments aimed at cross-complementing Ni-binding proteins required for urease and hydrogenase synthesis (UreE and HypB, respectively) indicated that these two proteins are not functionally interchangeable in R. leguminosarum.