Fig. 32.19. Use of transcription assembly factors (hatched) and transcription initiation factors (solid black) for binding of RNA polymerase I (Pol I) and RNA polymerase III (Pol III).
For the initiation of transcription at genes transcribed by RNA polymerase I
(Pol I) and RNA polymerase III
(Pol III), only one transcription factor each (TIFI and TFIIIB respectively), bound upstream, is needed for directing the RNA polymerase enzyme to its binding site (promoter). However, assembly factors may be needed for mediating the binding of transcription initiation factor. Therefore, following two types of frans-acting protein factors are needed for initiation by Pol I and Pol III : (i) assembly factors and (ij) initiataion factors. For RNA polymerase I (Pol I), the need for assembly factor (UBF) is not obligatory, but is species dependent. However, in
Xenopus laevis, for RNA polymerase III (Pol III), two assembly factors (TFIIIA and TFIIIC) in case of 5S RNA genes and one assembly factor (TFIIIC) for tRNA genes are needed to mediate the binding of transcription initiation factor, TFIIIB. These assembly factors can be removed, after binding of the initiation factor without any effect on subsequent transcription (5S RNA is an example). Role of initiation and assembly factors for Pol I and Pol III is shown in Fig 32.19.
Fig. 32.19. Use of transcription assembly factors (hatched) and transcription initiation factors (solid black) for binding of RNA polymerase I (Pol I) and RNA polymerase III (Pol III).
