Different binding of RNA polymerase to individual promoters

Summary

The amount of E. coli RNA polymerase which can be bound to individual promoters on λ pgal and λ dgal phage DNA in a stable heparin-resistant form was measured by assessing its capacity to transcribe, upon addition of the nucleoside triphosphates, the RNA sequences starting at these promoters. These RNA species were analysed by competition hybridization to separated single strands of λ, λpgal and λdgal phage DNA.

Individual promoters bind, at saturation, different numbers of polymerase molecules. From the amount of polymerase necessary to saturate all promoters (Fig. 3), from the proportions of RNA synthesized at the individual promoters (Table 1) and from the amounts of γ-³²P-ATP or-GTP label incorporated into the different RNA species (Tables 2 and 3) we calculate polymerase storage capacities for the promoters as follows: gal: 6 molecules; λ l-strand specific: 3–5 molecules starting with ATP and 1 molecule starting with GTP; λ r-strand specific: 3–5 molecules starting with ATP (and perhaps one molecule starting with GTP); these estimates are lower limits and may be too small by a factor of up to three.

The heparin resistant binding of six polymerase molecules to the gal promoter is dependent on CGA protein and cAMP, but ATP and GTP can allow one polymerase to bind to the same site or to a very close one.

Several parameters of polymerase binding are different with the individual promoters tested.