The role of the components sigma and y of the DNA-dependent RNA polymerase of Lactobacillus curvatus in promotor selection.

An average of 0.44 molecule each of the initiation factor sigma and the RNA polymerase binding protein y and 0.54 molecule of the subunit gamma per molecule of Lactobacillus curvatus DNA-dependent RNA polymerase have been found in the cell. Free factor y displaces sigma from free holo enzyme, E sigma. The formation of a binary complex from Ey, free sigma, and poly[d(A-T)], leads to immediate release of factor y. The release of the sigma factor occurs upon the transition of the binary to a ternary complex. A mixture of E and sigma forms binary complexes with all T7 DNA HpaII restriction fragments. In contrast a mixture of Ey and sigma binds selectively to promoter-containing DNA fragments, indicating that the stimulatory effect of y on transcription is due to an increase in the rate of promoter selection. The same RNA products are synthesised by E sigma and by Ey plus sigma with T7 DNA as template. Thus the nonspecific complexes formed by E sigma and T7 DNA are nonproductive. On the basis of these findings we propose a model for the transcription cycle in Lactobacillus curvatus.