Control of gal transcription through DNA looping: inhibition of the initial transcribing complex.

Involvement of DNA looping between two spatially separated gal operators, OE and OI, in repression of the gal operon has been demonstrated in vivo. An in vitro transcription assay using a minicircle DNA containing the gal promoter region with lac operators was employed to elucidate the molecular mechanism of repression. Wild-type lac repressors (LacI+ protein molecules), which are capable of associating into a tetramer and forming a DNA loop, repressed transcription from promoter sites P1 and P2, whereas a non-looping lac repressor mutant (LacI(adi)) failed to show normal repression of both of the gal promoters. Thus a DNA loop is also required for repression of transcription in vitro. Repression mediated by DNA looping resulted in the inhibition of the synthesis of complete as well as aborted transcripts, demonstrating that the repressive action was on the formation or activity of the initial transcribing complex. Under similar conditions, the gal repressor (GalR protein) did not repress the gal promoters effectively, apparently because it failed to loop DNA containing gal operators in the purified system. The component(s) or conditions that aid GalR in DNA looping remain to be identified.