RNA POLYMERASE

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.