Structure of RNA Polymerase

Subunits - E. coli RNA polymerase is a multi-subunit protein. The five distinct polypeptide subunits of E. coli RNA polymerase are summarized in Table 26.1. Two copies of the subunit are present, along with one each of , ', , and , giving an M r of about 450,000 for the holoenzyme. The subunit may be involved in regulation, but its precise role is not yet clear. It is not required for the reconstitution of active enzyme.

Subunit is the target for rifampicin inhibition and also plays a role in transcription initiation. is the subunit with the catalytic site for chain elongation.

The subunit plays an important role in directing RNA polymerase to bind to template at the proper site for initiation--the promoter site--and to select the correct strand for transcription. The addition of to core polymerase reduces the affinity of the enzyme for nonpromoter sites by about 104, thereby increasing the enzyme's specificity for binding to promoters. In at least some cases, gene expression is regulated by having core polymerase interact with different forms of , which would in turn direct the holoenzyme to different promoters.

Prokaryotic/eukaryotic RNA polymerases - RNA polymerases from different prokaryotic sources are remarkably similar in subunit size and composition. Eukaryotic RNA polymerases have much more complex and diverse subunit structures.

T7 RNA polymerase - Not all RNA polymerases in prokaryotic systems have multiple subunits. The best-known exception is RNA polymerase specified by bacteriophage T7. The left-hand 20% of the chromosome of its linear genome (as usually drawn) is transcribed early in infection by E. coli RNA polymerase. One of these early gene products is a virus-specified RNA polymerase. This single-subunit enzyme (Mr = 98,000) responds to different DNA control sequences and is responsible for all T7 transcription late in infection.

Recent crystallographic analysis of T7 RNA polymerase shows a DNA-binding domain similar to the "hand" structure seen in DNA polymerases, with a palm, fingers, and thumb that wrap about the DNA template

See also: RNA Polymerases, Interactions with Promoters, Initiation and Elongation, Factor-Independent Termination of Transcription, Factor-Dependent Termination of Transcription

INTERNET LINKS:

> 1. Eukaryotic Transcription

> 2. Regulation of Transcription by RNA Polymerase II

> 3. RNA Polymerase and GreA 3D Structures