Each of the two promoters of the gal operon is negatively regulated by two repressors, GalR and newly discovered GaIS. The repression by GalR is strong, whereas that of GalS is moderate, although both act by binding to the same two spatially separated operators, OE and OI. Additionally, GalR is epistatic to GalS; GalS-mediated repression is seen only when the cell is genetically devoid of GalR. The gene encoding GalS has been defined by isolating mutations in the galS gene and cloning the wild type and mutant genes. The amino acid sequence of GalS, as deduced from DNA sequence, is 85% similar to that of GalR. galR and galS themselves carry two operators, OE and 01, suggesting autoregulation by GalR and/or GalS through DNA looping -- the same way looping operates in the gal operon itself. The biochemical mechanism of negative control of the gal operon by GalR represssor has been studied in detail. We have proposed that repression involves """"""""caging"""""""" of RNA polymerase by a DNA loop formed by interaction of OE and OI bound repressors facilitating a transcription-inhibitory physical contact(s) between repressor and RNA polymerase. Consistent with the model, we have shown that (i) the specificity of the contact between operator-bound repressors is independent of the nature of DNA-protein interactions and resides in the carboxy domain of the repressor protein; (ii) the proposed repressor-RNA polymerase contact may be through the alpha-subunit of the holoenzyme; and (iii) transcription from both gal promoters in a unitary promoter plasmid minicircle is repressed in a purified system with wild type repressor. Repression was not observed with a mutant repressor that binds to the operators but does not loop the DNA.
