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
Mr 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
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