Promoter characterization and constitutive expression of the Escherichia coli gcvR gene

The Escherichia coli glycine cleavage repressor protein (GcvR) negatively regulates expression of the glycine cleavage operon (gcv). In this study, the gcvR translational start site was determined by N-terminal amino acid sequence analysis of a GcvR-LacZ fusion protein. Primer extension analysis of the gcvR promoter region identified a primary transcription start site 27 bp upstream of the UUG translation start site and a minor transcription start site approximately 100 bp upstream of the translation start codon. The -10 and -35 promoter regions upstream of the primary transcription start site were defined by mutational analysis. Expression of a gcvR-lacZ fusion was unaltered in the presence of glycine or inosine, molecules known to induce or repress expression of gcv, respectively. In addition, it was shown that gcvR-lacZ expression is neither regulated by the glycine cleavage activator protein (GcvA) nor autogenously regulated by GcvR. From DNA sequence analysis, it was predicted that the translation start codon of the downstream bcp gene overlaps the gcvR stop codon, suggesting that these genes may form an operon. However, a down mutation in the -10 promoter region of gcvR had no effect on the expression of a downstream bcp-lacZ fusion, and primer extension analysis of the bcp promoter region demonstrated that bcp has its own promoter within the gcvR coding sequence. These results show that gcvR and bcp do not form an operon. Furthermore, the deletion of bcp from the chromosome had no effect on gcv-lacZ expression.