To activate or destroy: regulation of the RNA Pol II elongation factor and ubiquitin ligase Elongin (550.3)

Abstract

Lesions induced by a variety of DNA damaging agents can block RNA polymerase II (Pol II) elongation. Mechanisms that reactivate or remove Pol II that has become stalled or arrested at such lesions are critical for resumption of gene expression after DNA damage repair and, in addition, may contribute to efficiency of the DNA repair process by enhancing access of the damage repair machinery to the lesion. Following exposure of cells to UV irradiation or other DNA damaging agents, arrested Pol II is ubiquitinated and degraded by the proteasome.Elongin was first identified and purified showing an activity that stimulates the rate at which Pol II elongates nascent transcripts in vitro and is composed of 3 subunits, Elongin A (EloA), B (EloB), and C (EloC). Although the majority of Elongin A isolated from cells contains the three subunit Elongin complex, it can also be linked to a heterodimeric module composed of the Cullin family protein Cul5 and the RING finger proteins Rbx1 or Rbx2 to form a larger complex that targets stalled Pol II for ubiquitination. A previous study used indirect immunofluorescence to provide evidence for increased nuclear colocalization of EloA and Cul5 after UV‐induced DNA damage, raising the possibility that DNA damage may promote assembly of the EloA ubiquitin ligase complex. Using a combination of MudPIT‐based mass spectrometry and FRET‐based approaches, we have obtained evidence that supports this model. In ongoing studies, we seek to define mechanisms by which stalled Pol II regulates assembly and activity of the Elo A ubiquitin ligase complex.