Selective effects of MgCl2 and temperature on the initiation of transcription at lac, gal, and lambda promoters.

We have studied the effect of Mg2+ on the formation of transcription preinitiation complexes (open complexes) at two adenosine 3':5'-monophosphate (cyclic AMP)-cyclic AMP receptor protein (CRP)-dependent promoters (lac and gal) and two phage lambda promoters, PL and PR. Mg2+ strongly interferes with open complex formation at the lac and gal promoters, partially inhibits the lambda PR promoter, and is without effect on the lambda PL promoter. Mutations in the lac and gal promoters can affect the response of the promoter to Mg2+. Cyclic AMP and CRP specifically reduce the inhibition of the lac and gal promoters by Mg2+. These factors also affect open complexes at lac and gal by (a) increasing the rate at which they are formed and (b) lowering the midpoint of the temperature transition curve for their formation by about 10 degrees. Open complexes at the lac promoter are more unstable to cooling, even in the presence of CRP and cyclic AMP, than open complexes at lambda promoters. Our studies suggest that the DNA of the lac and gal promoter regions is more resistant to denaturation than the DNA of phage promoter regions. Cyclic AMP and CRP act to decrease this stability, stimulating open complex formation under conditions unfavorable for DNA melting, e.g. low temperature and high Mg2+ concentrations.