SYNTHESIS OF THE BETA-SUBUNIT AND BETA'-SUBUNIT OF ESCHERICHIA-COLI RNA-POLYMERASE IS AUTOGENOUSLY REGULATED IN-VIVO BY BOTH TRANSCRIPTIONAL AND TRANSLATIONAL MECHANISMS

Numerous experiments have indicated that the synthesis of RNA polymera se (beta beta'alpha(2) sigma(70)) in Escherichia coli is autogenously regulated, In the present study, we have examined expression of the rp oB and rpoC genes which encode the beta and beta' subunits of RNA poly merase, These genes are the distal cistrons of the rplKAJLrpoBC riboso mal protein-RNA polymerase transcription unit, Both transcriptional (o peron) and translational (gene) fusions of either rpoB or rpoC to the lacZ reporter were used to monitor their in vivo expression by inserti ng single copies of these fusions into the bacterial chromosome on int egration-proficient lambda vectors, In order to examine the expression of the rpoBC genes in response to varied intracellular concentrations of the RNA polymerase subunits, a series of expression plasmids carry ing the rpoB, rpoC, rpoA (alpha) and rpoD (sigma(70)) genes in differe nt combinations were then introduced into these cells, Elevated concen trations of either beta or beta' were round to repress the expression of both rpoB and rpoC at the translational level. However, the simulta neous increase in the concentration of all the subunits that comprise holoenzyme repressed the transcription of rpoBC, To determine the site of this repression, additional operon fusions were utilized which pla ced lacZ downstream of each of the transcriptional regulatory sites of this gene cluster, including two promoters, rplKp and rplJp, and a tr anscriptional attenuator in the rplL-rpoB intercistronic region. Expre ssion from these fusions showed that transcriptional repression is ach ieved primarily by reducing initiation at both upstream promoters, cou pled with a small increase in termination at the attenuator.