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RNA polymerase Ⅲ dose not appear to require additional factors for transcription elongation or termination
Update time:2019-03-10 18:53:39   【 Font: Large  Medium Small

    Once the pre-initiation complex is assembled at the promoter, the players are in position to begin transcription elongation. The RNA polymerase Ⅲ transcription machinery melts the DNA flanking the transcription initiation site. TFIIIB is an active participant in this melting event. The first two nucleotides line up on the now exposed template strand and transcription elongation begins. Almost all of the RNA polymerase Ⅲ molecules escape from the promoter without significant pausing or arrest. Once a crystal structure becomes available for RNA polymerase Ⅲ,it will be interesting to compare it with that of RNA polymerase Ⅱ to see if there are obvious structure difference that explain why the two polymerase differ in their abilities to escape the promoter.
    The RNA polymerase Ⅲ transcription elongation complex moves at about the same rate as the RNA polymerase Ⅱ transcription elongation complex but, unlike the RNA polymerase Ⅱ transcription elongation complex, does not appear to appear to require any dedicated factors for transcription elongation. However, a subunit that is unique to RNA polymerase Ⅲ has significant homology to the elongation factor SII. Perhaps this polymerase subunit eliminates the need for SII.
    The bound transcription initiation factors might be expected to block transcription elongation through transcription units with type 1 or 2 promoters. However, this blocking does not seem to occur. Moreover, multiple passages of RNA polymerase through a transcription unit do not remove the assembled transcription initiation factors from the transcription unit. One possibility is that RNA polymerase Ⅲ transiently displace one transcription factor as it moves through the transcription unit but protein- protein interactions with other transcription initiation factors permit the displaced factor to remain associated with the promoter.
    RNA polymerase Ⅲ can efficiently terminate transcription in the absence of others factors under in vitro conditions. Simple clusters of four or more U residues usually suffice as a termination signal. Further studies are required to determine whether specific termination factors are required in vivo.

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