Ureolytic clinical isolates of Providencia stuartii, Salmonella spp., and some Escherichia coli strains contain large urease-encoding plasmids. Expression of urease activity from these isolates is induced at least 20-fold by urea. In order to facilitate studies on the regulatory mechanism controlling this urea-inducible expression, the plasmid-encoded urease genes were inserted into the low-copy-number vector pRK415, to form pSEF70. Deletion mutagenesis of pSEF70 demonstrated that between 1.3 and 1.6 kb of DNA upstream of ureD (the first of seven urease genes clustered in an operon-like fashion) was required for a urease-positive phenotype. An open reading frame coding for a 34.1-kDa polypeptide was found in the DNA sequence of this upstream region. This open reading frame has been designated ureR, for urease regulator. A urea-inducible promoter region was identified upstream of ureD. Transcription from this promoter was activated only when ureR was present in trans. The predicted ureR gene product contains a helix-turn-helix motif and shows significant amino acid similarity to the AraC family of transcriptional activators. We conclude that urea-dependent expression from the plasmid-encoded urease gene cluster requires ureR and that ureR codes for a positive regulatory element controlling transcription of at least one essential urease gene, ureD.