text: The eukaryotic core RNA polymerase II was first purified using transcription assays. The purified enzyme has typically 10–12 subunits (12 in humans and yeast) and is incapable of specific promoter recognition. Many subunit-subunit interactions are known.
  * DNA-directed RNA polymerase II subunit RPB1 – an enzyme that in humans is encoded by the POLR2A gene and in yeast is encoded by RPO21. RPB1 is the largest subunit of RNA polymerase II. It contains a carboxy terminal domain (CTD) composed of up to 52 heptapeptide repeats (YSPTSPS) that are essential for polymerase activity. The CTD was first discovered in the laboratory of C.J. Ingles at the University of Toronto and by JL Corden at Johns Hopkins University. In combination with several other polymerase subunits, the RPB1 subunit forms the DNA binding domain of the polymerase, a groove in which the DNA template is transcribed into RNA. It strongly interacts with RPB8.
  * RPB2 (POLR2B) – the second-largest subunit that in combination with at least two other polymerase subunits forms a structure within the polymerase that maintains contact in the active site of the enzyme between the DNA template and the newly synthesized RNA.
  * RPB3 (POLR2C) – the third-largest subunit. Exists as a heterodimer with another polymerase subunit, POLR2J forming a core subassembly. RPB3 strongly interacts with RPB1-5, 7, 10–12.
  * RNA polymerase II subunit B4 (RPB4) – encoded by the POLR2D gene is the fourth-largest subunit and may have a stress protective role.
  * RPB5 – In humans is encoded by the POLR2E gene. Two molecules of this subunit are present in each RNA polymerase II. RPB5 strongly interacts with RPB1, RPB3, and RPB6.
  * RPB6 (POLR2F) – forms a structure with at least two other subunits that stabilizes the transcribing polymerase on the DNA template.
  * RPB7 – encoded by POLR2G and may play a role in regulating polymerase function. RPB7 interacts strongly with RPB1 and RPB5.
  * RPB8 (POLR2H) – interacts with subunits RPB1-3, 5, and 7.
  * RPB9 – The groove in which the DNA template is transcribed into RNA is composed of RPB9 (POLR2I) and RPB1.
  * RPB10 – the product of gene POLR2L. It interacts with RPB1-3 and 5, and strongly with RPB3.
  * RPB11 – the RPB11 subunit is itself composed of three subunits in humans: POLR2J (RPB11-a), POLR2J2 (RPB11-b), and POLR2J3 (RPB11-c).
  * RPB12 – Also interacts with RPB3 is RPB12 (POLR2K).