in the antiparallel direction. The initial product is complementary to
the region of the DNA employed as template and the RNA may be modified
9
post transcriptionally.
Prokaryotic Transcriptions
Background
There are several reviews of prokaryotic transcription available
(35,36,56,57) which may be consulted for discussions of aspects that
will not be considered here. The basic transcriptional studies have
been carried out in bacterial systems with and without phage infection
and these studies have served as a model system for understanding the
transcriptional process and its control. Only the properties of the
bacterial RNA polymerase and the regulation of bacterial RNA synthesis
will be reviewed here.
Prokaryotic RNA polymerase
The bacterial RNA polymerases are large molecules (molecular
weights between 400,000 and 500,000) and have complex subunit structures.
Two enzymatically active forms of the RNA polymerase are currently known:
the holoenzyme and the core enzyme. The holoenzyme contains the following
polypeptide chain subunits: one beta prime (3') subunit; one beta (3)
subunit; two alpha (a) subunits; and either one sigma (o) subunit or one
sigma prime (o') subunit. The subunits have the molecular weights of
approximately 160,000; 155,000; 90,000; and 40,000 respectively. Two
forms of the holoenzyme with the structures ct233'ci(67) and a233'o'(68)
have been found in Escherichia coli. The core enzyme lacks a sigma
subunit. Therefore, the holoenzyme can be separated into two functional
parts: a core enzyme, which is able to synthesize RNA but lacks the