Mechanism of the rifampicin induction of RNA polymerase beta and beta' subunit synthesis in Escherichia coli.

The mechanism of the rifampicin induction of RNA polymerase beta and beta' subunit synthesis was investigated, employing an in vitro coupled system of transcription and translation as well as an in vitro transcription system with purified RNA polymerase holoenzyme. Two independent effects leading to the induction of beta and beta' polypeptide synthesis have been identified for the drug: 1) rifampicin binds to RNA polymerase holoenzyme and inhibits its autorepressor activity, thus relieving the autorepression exerted possibly on the translation of rpoBC mRNA; and 2) rifampicin-RNA polymerase complex functions as a positive effector stimulating the transcription of rpoBC genes, but not of rplL gene. Using a terminally labeled DNA fragment covering the intercistronic region between rplL and rpoB as a probe, the structure of both in vivo and in vitro RNA were analyzed by S1 nuclease-mapping assays. The experimental results indicated that the observed stimulation of the transcription by rifampicin was attributed, at least in part, to relaxation of the transcription attenuation immediately preceding the rpoB gene, resulting in the increase of the read-through transcript to rpoBC genes. Besides the read-through transcript and the processed mature rpoBC mRNA, another RNA species was found in in vivo RNA, which might be transcribed from a weak promoter located in the intercistronic region. However, no clear indication has been obtained of possible enhancement of the transcription initiation by rifampicin from the putative promoter.