DNA-dependent RNA polymerase catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Largest and catalytic component of RNA polymerase II which synthesizes mRNA precursors and many functional non-coding RNAs. Forms the polymerase active center together with the second largest subunit. Pol II is the central component of the basal RNA polymerase II transcription machinery. During a transcription cycle, Pol II, general transcription factors and the Mediator complex assemble as the preinitiation complex (PIC) at the promoter. 11-15 base pairs of DNA surrounding the transcription start site are melted and the single-stranded DNA template strand of the promoter is positioned deeply within the central active site cleft of Pol II to form the open complex. After synthesis of about 30 bases of RNA, Pol II releases its contacts with the core promoter and the rest of the transcription machinery (promoter clearance) and enters the stage of transcription elongation in which it moves on the template as the transcript elongates. Pol II appears to oscillate between inactive and active conformations at each step of nucleotide addition. Elongation is influenced by the phosphorylation status of the C-terminal domain (CTD) of Pol II largest subunit (RPB1), which serves as a platform for assembly of factors that regulate transcription initiation, elongation, termination and mRNA processing. Pol II is composed of mobile elements that move relative to each other. The core element with the central large cleft comprises RPB3, RBP10, RPB11, RPB12 and regions of RPB1 and RPB2 forming the active center. The clamp element (portions of RPB1, RPB2 and RPB3) is connected to the core through a set of flexible switches and moves to open and close the cleft. A bridging helix emanates from RPB1 and crosses the cleft near the catalytic site and is thought to promote translocation of Pol II by acting as a ratchet that moves the RNA-DNA hybrid through the active site by switching from straight to bent conformations at each step of nucleotide addition. In elongating Pol II, the lid loop (RPB1) appears to act as a wedge to drive apart the DNA and RNA strands at the upstream end of the transcription bubble and guide the RNA strand toward the RNA exit groove located near the base of the largely unstructured CTD domain of RPB1. The rudder loop (RPB1) interacts with single-stranded DNA after separation from the RNA strand, likely preventing reassociation with the exiting RNA. The cleft is surrounded by jaws: an upper jaw formed by portions of RBP1, RPB2 and RPB9, and a lower jaw, formed by RPB5 and portions of RBP1. The jaws are thought to grab the incoming DNA template, mainly by RPB5 direct contacts to DNA. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Component of the RNA polymerase II (Pol II) complex consisting of 12 subunits. Interacts with ASK10, ESS1, RTT103 and SHE2. The C-terminal domain (CTD) serves as a platform for assembly of factors that regulate transcription initiation, elongation, termination and mRNA processing. The tandem 7 residues repeats in the C-terminal domain (CTD) can be highly phosphorylated. The phosphorylation activates Pol II. Phosphorylation occurs mainly at residues 'Ser-2' and 'Ser-5' of the heptapeptide repeat. The phosphorylated form of Pol II appears to carry, on average, one phosphate per repeat. The phosphorylation state is believed to result from the balanced action of site-specific CTD kinases and phosphatases, and a 'CTD code' that specifies the position of Pol II within the transcription cycle has been proposed. Phosphorylation at 'Ser-5' occurs in promoter-proximal regions in early elongation. Phosphorylation at 'Ser-2' predominates in regions more distal to the promoter and triggers binding of the 3' RNA processing machinery. CTD kinases include KIN28 (as part of the TFKII complex, a subcomplex of the TFIIH holo complex), SSN3/SRB10 (as part of the SRB8-11 complex, a module of the Mediator complex), CTK1 (as part of CTD kinase), and probably BUR1 (as part of the BUR1-BUR2 kinase complex). Phosphatases include FCP1 and SSU72. Mutagenesis experiments demonstrate that the minimum viable CTD contains eight consensus Y-S-P-T-S-P-[A-S-N-G] heptapeptide repeats. Identical and simultaneous substitutions in a number of consecutive repeats are lethal: 'Ser-2' -> 'Ala-2' (14 repeats), 'Ser-5' -> 'Ala-5' (15 repeats), '2-Ser-Pro-Thr-Ser-5'-> '2-Ala-Pro-Thr-Ala-5' (10 repeats), 'Ser-2'-> 'Glu-2' (15 repeats), 'Ser-5' -> 'Glu-5' (12 repeats), '2-Ser-Pro-3' -> '2-Pro-Ser-3' (15 repeats) and 'Tyr-1' -> 'Phe-1' (12 repeats). The binding of ribonucleoside triphosphate to the RNA polymerase II transcribing complex probably involves a two-step mechanism. The initial binding seems to occur at the entry (E) site and involves a magnesium ion temporarily coordinated by three conserved aspartate residues of the two largest RNA Pol II subunits. The ribonucleoside triphosphate is transferred by a rotation to the nucleotide addition (A) site for pairing with the template DNA. The catalytic A site involves three conserved aspartate residues of the RNA Pol II largest subunit which permanently coordinate a second magnesium ion. Belongs to the RNA polymerase beta' chain family.