Proteus mirabilis urease: genetic organization, regulation, and expression of structural genes

Proteus mirabilis, a cause of serious urinary tract infection, produces urease, an important virulence factor for this species. The enzyme hydrolyzes urea to CO2 and NH3, which initiates struvite or apatite stone formation. Genes encoding urease were localized on a P. mirabilis chromosomal DNA gene bank clone in Escherichia coli by deletion analysis, subcloning, Bal31 nuclease digestion, transposon Tn5 mutagenesis, and in vitro transcription-translation. A region of DNA between 4.0 and 5.4 kilobases (kb) in length was necessary for urease activity and was located within an 18.5-kb EcoRI fragment. The operon was induced by urea and encoded a multimeric, cytoplasmic enzyme comprising subunit polypeptides of 8,000, 10,000, and 73,000 daltons that were encoded by a single polycistronic mRNA and transcribed in that order. Seventeen urease-negative transposon insertions were isolated that synthesized either none of the structural subunit polypeptides, the 8,000-dalton polypeptide alone, or both the 8,000- and 10,000-dalton subunit polypeptides. The molecular weight of the native enzyme was estimated to be 212,000 by Superose-6 chromatography. Homologous sequences encoding the urease of Providencia stuartii synthesized subunit polypeptides of similar sizes and showed a similar genetic arrangement. However, restriction maps of the operons from the two species were distinct, indicating significant divergence.