After assembly of the complete RNA polymerase II-preinitiation complex, the next step is separation of the two DNA strands. This isomerization step is known as the closed-to-open complex transition and occurs prior to the initiation of mRNA synthesis. In the RNA polymerase II system this step requires the hydrolysis of ATP or dATP into Pi and ADP or dADP (in contrast to the other RNA polymerase systems) and is catalyzed by the XPB subunit of TFIIH. The region of the promoter, which becomes single-stranded , spans from –10 to +2 relative to the transcription start site.
Negative supercoiling in the promoter region probably induces transient opening events and can alleviate requirement of TFIIE, TFIIH and ATP-hydrolysis for open complex formation. ATP is also used in this step by the cdk7-subunit of TFIIH to phosphorylate the heptad repeats of the C-terminal domain of the largest subunit of RNA polymerase II (RPB1) on serine-2