Group of enzymes for catalyzing the synthesis of RNA from a DNA template (sense
strand). Bacteria possess only a single type of RNA polymerases, but eukaryotic cells possess at least four. They are
designated: type I (pol I), which is responsible for synthesis of
ribosomal RNA (except 5S RNA); type II (pol II), which is responsible for synthesis of messenger RNA (and its precursor,
heterogenous nuclear RNA); type III (pol III), which is responsible for synthesis of transfer RNA and the small ribosomal 5
SRNA; and type IV, which is the RNA polymerase of the
mitochondria. RNA polymerases are sensitive to various antibiotics; for example, pol II is sensitive to a-amanitin and pol I
to actinomycin D. Since a single species of RNA polymerase
is responsible for synthesis of all messenger RNA, the specificity of gene expression is clearly not determined by the polymerase enzyme, but since the promoter sequence (see
promoter) upstream of the initiation site of the coding
sequence is recognized by the enzyme, differences in the
sequence of the promoter between different genes will affect
transcription rates. RNA polymerases are complex proteins.
The bacterial enzyme consists of a core enzyme comprising a-,
b-, and b’-subunits, and a sigma factor, which recognizes the
pribnow box.
