DNA-dependent RNA polymerase catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Component of RNA polymerase II which synthesizes mRNA precursors and many functional non-coding RNAs. Pol II is the central component of the basal RNA polymerase II transcription machinery. It is composed of mobile elements that move relative to each other. RPB3 is part of the core element with the central large cleft and the clamp element that moves to open and close the cleft. Component of the RNA polymerase II (Pol II) core complex consisting of 12 subunits: a ten-subunit catalytic core composed of POLR2A/RPB1, POLR2B/RPB2, POLR2C/RPB3, POLR2I/RPB9, POLR2J/RPB11, POLR2E/RPABC1, POLR2F/RPABC2, POLR2H/RPABC3, POLR2K/RPABC4 and POLR2L/RPABC5 and a mobile stalk composed of two subunits POLR2D/RPB4 and POLR2G/RPB7, protruding from the core and functioning primarily in transcription initiation. Part of Pol II (G) complex, in which Pol II core associates with an additional subunit POLR2M; unlike conventional Pol II, Pol II (G) functions as a transcriptional repressor. Part of TBP-based Pol II pre-initiation complex (PIC), in which Pol II core assembles with general transcription factors and other specific initiation factors including GTF2E1, GTF2E2, GTF2F1, GTF2F2, TCEA1, ERCC2, ERCC3, GTF2H2, GTF2H3, GTF2H4, GTF2H5, GTF2A1, GTF2A2, GTF2B and TBP; this large multi-subunit PIC complex mediates DNA unwinding and targets Pol II core to the transcription start site where the first phosphodiester bond forms.