| Comments |
"Three different RNA polymerase (pol) complexes
perform RNA synthesis in the nuclei of mammalian
cells (11, 12). In most cells, RNA polymerase II (pol
II) is the major activity, transcribing all protein-coding genes to generate patterns of gene expression that determine cell type. Synthesis is performed
by an
~4 MDa holoenzyme containing the pol II
core enzyme and other activities required during
RNA synthesis and processing (primary sources). RNA polymerase I (pol I) is dedicated to the synthesis of the
repeated ribosomal RNA (rRNA) genes, within specialized nuclear sites—nucleoli—and RNA polymerase III (pol III), a minor nucleoplasmic activity,
transcribes transfer RNA (tRNA) and 5S rRNA
genes. Small nuclear RNA (snRNA) and small nucleolar RNA (snoRNA) genes encode structural RNAs
needed for RNA processing some are transcribed by
pol II and others by pol III." |