DNA-directed RNA pol I, largest subunit <p>DNA-dependent RNA polymerases (<db_xref db="EC" dbkey="2.7.7.6"/>) are responsible for the polymerisation of ribonucleotides into a sequence complementary to the template DNA. In eukaryotes, there are three different forms of DNA-dependent RNA polymerases transcribing different sets of genes. Most RNA polymerases are multimeric enzymes and are composed of a variable number of subunits. RNA synthesis follows after the attachment of RNA polymerase to a specific site, the promoter, on the template DNA strand. The RNA synthesis process continues until a termination sequence is reached. The RNA product, which is synthesised in the 5' to 3' direction, is known as the primary transcript.</p> <p>Eukaryotic nuclei contain three distinct types of RNA polymerases that differ in the RNA they synthesise:</p> <p>1. RNA polymerase I: located in the nucleoli, synthesises precursors of most ribosomal RNAs.</p> <p>2. RNA polymerase II: occurs in the nucleoplasm, synthesises mRNA precursors.</p> <p>3. RNA polymerase III: also occurs in the nucleoplasm, synthesises the precursors of 5S ribosomal RNA, the tRNAs, and a variety of other small nuclear and cytosolic RNAs.</p> <p>Eukaryotic cells are also known to contain separate mitochondrial and chloroplast RNA polymerases. Eukaryotic RNA polymerases, whose molecular masses vary in size from 500 to 700 kDa, contain two non-identical large (greater than 100 kDa) subunits and an array of up to 12 different small (less than 50 kDa) subunits.</p> <p>This entry appears to be specific to RNA polymerase I, largest subunit.</p>