GalR-mediated repression and activation of hybrid lacUV5 promoter: differential contacts with RNA polymerase

The GalR repressor regulates expression of genes of the gal regulon in Esch erichia coli. We studied the regulatory effect of GalR in vitro on a hetero logous promoter, lacUV5, by placing the GalR-binding site, O-E, at differen t locations upstream of this promoter. Despite the fact that the lacUV5 pro moter is transcribed efficiently by RNA polymerase (RNP) alone, GalR modula ted transcription from many of the PlacUV5 variants. Depending on the locat ion of O-E and the neighboring DNA sequence, GalR repressed, activated or h ad no effect on the promoter. Both repression and activation involved forma tion of GalR-RNP-DNA ternary complexes and required an intact c-domain of t he alpha subunit of the holoenzyme. These results support the differential contact model of a regulator action, in which a regulator differentially bi nds to, and lowers the energy of, intermediates of transcription initiation either to hinder or to facilitate a step of initiation. The nature of the contacts depends upon the context, i.e. the geometry of the ternary complex . The observed repression and activation effect of GalR on a heterologous p romoter also underscores the point that a regulator is not a dedicated prot ein for repression or for activation. (C) 1998 Published by Elsevier Scienc e B.V. All rights reserved.