60 Cook and Slater
template and interact with protein factors that modulate the activity
of the enzyme. Several reviews have appeared on eukaryotic RNA
polymerases (2-4).
- Biological Function
Bacterial RNA polymerase containing a single apo enzyme is respon-
sible for all the synthesis of RNA in the cell. There are approx 7000
RNA polymerase molecules present in an Escherichia coli cell, and
many of these are involved in transcription, although the actual num-
ber depends on the growth conditions (5). The RNA polymerase best
characterized is that of E. coli, but its structure is similar in all other
bacteria studied. The holoenzyme has a mol mass of about 480,000 Da
and is made up of five subunits, a, 13, 13', o, and m, there being two
copies of o~ and one each of the others: ~x21313'o¢0. The holoenzyme can
be separated into two components, the apo enzyme (a21313'o)) and the
sigma factor (o). The sigma subunit of the E. coli polymerase has a
specific role as an initiation factor for transcription: It enables the
enzyme to find consensus promoter sequences. Only the holoenzyme
can initiate transcription; the sigma "factor" is then released, leaving
the core enzyme to undertake elongation. Thus, the core enzyme has
the ability to synthesize RNA on a DNA template; its affinity for DNA
is the result of electrostatic attractions, but is nonspecific. It is the
sigma factor that ensures correct transcription initiation involving the
stable binding of the RNA polymerase to the promoter (5). Eukary-
otes, however, contain multiple DNA-dependent RNA polymerases.
Those that transcribe nuclear genomes are generally referred to as
RNA polymerases I, II, and III transcribing rRNA, mRNA and tRNA,
and 5SrRNA, respectively (Table 1).
2.1. Properties
Eukaryotic RNA polymerases transcribe both double-stranded and
single-stranded DNA templates in vitro, and single-stranded templates
generally promote higher rates of transcription (6). RNA polymerases
of plants and animals are capable of initiating transcription at nicks in
the DNA template in vitro (3, 8). Purified eukaryotic RNA polymerases
are capable of some degree of selective binding and initiation of RNA
chains on deproteinized DNA templates (9,10), although this is non-
specific, and in no case has the selectivity been correlated with a
promoter or initiation site. From a number of studies with animal cell-free
