Mutations in the CSA or CSB complementation genes cause the Cockayne syndrome, a severe genetic disorder that results in patients’ death in early adulthood. CSA and CSB act in a transcription-coupled repair (TCR) pathway, but their functional relationship is not understood. We have previously shown that CSA is a subunit of an E3 ubiquitin ligase complex. Here we demonstrate that CSB is a substrate of this ligase: Following UV irradiation, CSB is degraded at a late stage of the repair process in a proteasome- and CSA-dependent manner. Moreover, we demonstrate the importance of CSB degradation for post-TCR recovery of transcription and for the Cockayne syndrome. Our results unravel for the first time the functional relationship between CSA and CSB.

Terminal differentiation of muscle cells results in opposite effects on gene promoters: muscle-specific promoters, which are repressed during active proliferation of myoblasts, are turned on, whereas at least some proliferation-associated promoters, such as c-fos, which are active during cell division, are turned off. MyoD and myogenin, transcription factors from the basic-helix-loop-helix (bHLH) family, are involved in both processes, up-regulating muscle genes and down-regulating c-fos. On the other hand, the serum response factor (SRF) is involved in the activation of muscle-specific genes, such as c-fos, as well as in the up-regulation of a subset of genes that are responsive to mitogens. Upon terminal differentiation, the activity of these various transcription factors could be modulated by the formation of distinct protein-protein complexes. Here, we have investigated the hypothesis that the function of SRF and/or MyoD and myogenin could be modulated by a physical association between these transcription factors. We show that myogenin from differentiating myoblasts specifically binds to SRF. In vitro analysis, using the glutathione S-transferase pull-down assay, indicates that SRF-myogenin interactions occur only with myogenin-E12 heterodimers and not with isolated myogenin. A physical interaction between myogenin, E12, and SRF could also be demonstrated in vivo using a triple-hybrid approach in yeast. Glutathione S-transferase pull-down analysis of various mutants of the proteins demonstrated that the bHLH domain of myogenin and that of E12 were necessary and sufficient for the interaction to be observed. Specific binding to SRF was also seen with MyoD. In contrast, Id, a natural inhibitor of myogenic bHLH proteins, did not bind SRF in any of the situations tested. These data suggest that SRF, on one hand, and myogenic bHLH, on the other, could modulate each other's activity through the formation of a heterotrimeric complex.