Characterization of the Escherichia coli gcvR gene encoding a negative regulator of gcv expression

The Escherichia coli glycine cleavage enzyme system catalyzes the cleavage of glycine, generating CO2, NH3, and a one-carbon unit. Expression of the operon encoding this enzyme system (gcv) is induced in the presence of glycine and repressed in the presence of purines. In this study, a mutant with high-level constitutive expression of a gcvT-lacZ gene fusion was isolated. The mutation in this strain was designated gcvR1 and was mapped to min 53.3 on the E. coli chromosome. A single-copy plasmid carrying the wild-type gcvR gene complemented the mutation, restoring normal regulation of a gcvT-lacZ fusion, while a multicopy plasmid carrying gcvR led to superrepression under all growth conditions. Negative regulation of a gcvT-lacZ fusion by GcvR was shown to require GcvA, a LysR family protein known to both activate gcv in the presence of glycine and repress gcv in the presence of purines. Models explaining how GcvR and GcvA might interact to regulate gcv expression are proposed.