The formation of active urease in Kiebsiella aerogenes requires the presence ofthree structural genes for the apoprotein (ureA, ureB, and ureC), as well as four accessory genes (ureD, ureE, ureF, and ureG) that are involved in functional assembly of the metallocenter in this nickelcontaining enzyme. Slow and partial activation of urease apoprotein was observed after addition of nickel ion to extracts of Escherichia coli cells bearing a plasmid contining the K. aerogenes urease gene cluster or derivatives ofthis plasmid with deletions in ureE, ureF, or ureG. In contrast, extracts of cells containing a ureD deletion derivative failed to generate active urease, thus highlighting a key role for UreD in the metallocenter assembly process. Site-directed mutagenesis methods were used to overexpress ureD in the presence of the other urease genes, and the UreD protein was found to copurify with urease.

In vitro studies on the processing of halobacterial tRNA introns have led to the proposal that archaeal and eukaryotic tRNA intron endonucleases have distinctly different requirements for the recognition of pre-tRNAs. Using a Haloferax volcanii in vivo expression vector we have examined the in vivo processing of modified forms of the halobacterial intron-containing tRNA(Trp) gene. As observed in vitro, changes in the exon-intron boundary structure of this pre-tRNA block processing. Intron sequences, other than those at the exon-intron boundaries, are not essential for processing in vivo. We also show that conversion of the tryptophan anticodon to an opal suppressor anticodon is tolerated when the exon-intron boundary structure is maintained.