Function and Reassembly of Subunits of DNA-dependent RNA Polymerase

Abstract

INTRODUCTION
DNA-dependent RNA polymerases of prokaryotes are multisubunit enzymes. With one exception (Louis and Fitt 1971), holoenzyme monomers from the different sources thus far analyzed contain two large subunits, β′ and β (MW about 160,000), two identical or at least almost identical small subunits, α (MW about 40,000), and an initiation factor, σ normally of intermediate molecular weight (ranging from 44,000 to 92,000 in the different RNA polymerases known to date) (see, for instance, Lill, Behrendt and Hartmann 1975; Palm et al. 1975). Core enzyme is obtained by the removal of factor σ from holoenzyme (Burgess et al. 1969). Thus the composition formulas of holoenzyme and core enzyme are β′βα₂σ and β′βα₂, respectively.

This complexity in structure is paralleled by the complexity of the transcription process catalyzed by RNA polymerase. The transcription cycle can be formally subdivided into several steps, including binding of the enzyme to the template, initiation steps that yield a tight complex of RNA polymerase with the promoter and allow the formation of the first internucleotide bond, followed by elongation steps, and finally termination, in which the product (RNA) is liberated and the enzyme becomes available for the next cycle. At least two recognition events involving specific interaction between RNA polymerase and DNA signals are essential for the process: first, in binding and initiation between RNA polymerase and promoter, and second, in termination between RNA polymerase and terminator. Evidence has been obtained for the existence of several active centers or sites in the enzyme, e.g., two sites...