Formation of preinitiation (transcription) complex with RNA polymerase II (Pol II)
The promoter sequence responsible for constitutive expression of common genes (also called house keeping genes) in all cells is called generic promoter, which can not bring about regulated expression (tissue or stimulus specific). Initiation of transcription on this generic promoter by RNA polymerase II requires the action of several transcription factors (variously designated in humans, rat and yeast; Table 32.3) in the following defined order (Fig. 32.16) : (i) TFIID binds at the TATA box, which permits (ii) the association of TFIIA and TFIIB; (iii) only after binding of TFHB, forming the so called DB complex, RNA polymerase associates to promoter site; (iv) RNA polymerase is escorted to the promoter by TFIIF to form a transcription complex; (v) orderly addition of TFIIE, TFIIH and TFIIJ helps in the initiation. The requirement of TFIIA and TFIIE has not been found universal. The function of TFIIA can be performed by TFIIG or TFII I (two factors discovered in 1990's); TFIIE, on the other hand, though needed for adenovirus (Ad) ML, H2B and H4 prmoters, has not been found essential for immunoglobulin heavy chain gene (Ig H) promoter. Besides the above transcription factors, hundreds of other transcription factors have been discovered, which are needed for tissue or stimulus specific transcription.