Genes encoding Escherichia coli aspartate transcarbamoylase: the pyrB-pyrI operon.

In both Escherichia coli and Salmonella typhimurium there is approximately balanced synthesis of the six catalytic and six regulatory polypeptide chains of the regulatory enzyme aspartate transcarbamoylase (carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2). This control is achieved by the contiguous pyrB and pyrI genes, which encode the catalytic and regulatory chains, respectively. Evidence for this single transcriptional unit was obtained by a study of various deletion mutations and from the DNA sequence of the intercistronic region between pyrB and pyrI. One pyrB deletion, pyrB748, produced a normal level of regulatory chains even though it removed a substantial portion of the pyrB gene. Another deletion, pyrB740, shares a similar terminus at one end within pyrB, but the promoter region is removed. In this deletion mutation, there is no production of the regulatory polypeptide, indicating that a single region adjacent to pyrB controls transcription of pyrI as well. Molecular cloning and subsequent DNA analysis demonstrated that the pyrB and pyrI genes are contiguous with pyrI as the distal gene in the operon. The cistrons are separated by a 15-nucleotide untranslated region containing a sequence capable of interacting with the 16S ribosomal RNA and allowing translation of the pyrI cistron.