Results and Discussion

Urease synthesis requires the concerted action of at least seven proteins, the products of the ure genes (Mobley et al. 1995). An R. leguminosarum bv. viciae DNA region of ca. 9 kb containing the urease structural genes (ureA, ureB and ureC), accessory genes (ureD, ureE, ureF and ureG), and 5 additional orfs (orjS3, orfl35, orf207, orf223 and orf287) encoding proteins of unknown function has been sequenced. Three of these orfs (orf83, orfl35 and orf207) were conserved in the Sinorhizobium meliloti urease gene cluster (Miksch et al. 1994). orf287 encodes a potential transmembrane protein with a C-terminal GGDEF domain, for which a nucleotidyl cyclase activity has been proposed (Pei, Grishin 2001). A mutant affected in orf287 exhibited normal levels of urease activity in vegetative cells. R. leguminosarum UreE contains a long histidine-rich C-terminus (24 of the last 57 residues). A mutant strain affected in ureE gene was partially complemented by nickel addition to the medium, suggesting the involvement of this protein in nickel provision for urease.

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