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P.338 left column:"Gene transcription in eukaryotic cells is carried out by the three different DNA-dependent RNA polymerases Pol I, Pol II, and Pol III. Pol I produces ribosomal RNA, Pol II synthesizes messenger RNAs and small nuclear RNAs, and Pol III produces transfer RNAs and other small RNAs. A fourth RNA polymerase, Pol IV, which was recently discovered in plants, is not included here, as its composition and structure are currently unknown. The RNA polymerases are multisubunit enzymes. Pol I, II, and III comprise 14, 12, and 17 subunits, respectively, and have a total molecular weight of 589, 514, and 693 kDa, respectively. Ten subunits form a structurally conserved core, and additional subunits are located on the periphery." For subunit structure see p.339 table 1 - link |