Studies on deo operon regulation in Escherichia coli: cloning and expression of the cytR structural gene

The structural gene that encodes one repressor (the cytR-encoded repressor) of the Escherichia coli deo operon has been cloned from a lambda dmet transducing phage into the multicopy plasmid pBR322 by selecting for ApR, Lac- transformants of E. coli SS110(delta lac, cytR, tsx::lac). Restriction maps for the cytR+ plasmids have been generated and the position of the cytR gene on the cloned insert of these plasmids has been determined through deletion analysis. Results from maxicell experiments employing pCB001 and its cytR- derivatives suggest that the cytR gene encodes a protein with a subunit Mr of 37 000. In contrast to the complete repression of the deo operon obtained when deoR+ plasmids were introduced into E. coli SS201 (deoR, cytR), transformation of this DeoR-, CytR- strain with any of the cytR+ plasmids yields only clones which have phenotypes and Deo enzyme levels characteristic of a DeoR- single mutant. The data presented in this study are consistent with the interpretation that, in E. coli, the deoR-encoded repressor controls deo operon transcription initiating from both deo promoter-operator sites, PO1 and PO2. In contrast, the cytR-encoded repressor regulates deo operon expression only through deo promoter-operator site PO2.