RNA Polymerase II exists in two forms unphosphorylated and phosphorylated, IIA and IIO respectively. The transition between the two forms facilitates different functions for transcription. The phosphorylation of CTD is catalyzed by one of the six general transcription factors, TFIIH. TFIIH serves two purposes: one is to unwind the DNA at the transcription start site and the other is to phosphorylate. The form polymerase IIA joins the preinitiation complex, this is suggested because IIA binds with higher affinity to the TBP (TATA-box binding protein), the subunit of the general transcription factor TFIID, than polymerase IIO form. The form polymerase IIO facilitates the elongation of the RNA chain. The method for the elongation initiation is done by the phosphorylation of Serine at position 5 (Ser5), via TFIIH. The newly phosphorylated Ser5 recruits enzymes to cap the 5 end of the newly synthesized RNA and the "3 processing factors to poly(A) sites". Once the second Serine is phosphorylated, Ser2, elongation is activated. In order to terminate elongation dephosphorylation must occur. Once the domain is completely dephosphorylated the RNAP II enzyme is "recycled" and catalyzes the same process with another initiation site.