In eukaryotic cells, transcription coupled nucleotide excision repair (TCR) is believed to be initiated by RNA polymerase II (Pol II) stalled at a lesion in the transcribed strand of a gene. Rad26, the yeast homolog of the human Cockayne syndrome group B (CSB) protein, plays an important role in TCR. Spt4, a transcription elongation factor that forms a complex with Spt5, has been shown to suppress TCR in rad26⌬ cells. Here we present evidence that Spt4 indirectly suppresses Rad26-independent TCR by protecting Spt5 from degradation and stabilizing the interaction of Spt5 with Pol II. We further found that the C-terminal repeat (CTR) domain of Spt5, which is dispensable for cell viability and is not involved in interactions with Spt4 and Pol II, plays an important role in the suppression. The Spt5 CTR is phosphorylated by the Bur kinase. Inactivation of the Bur kinase partially alleviates TCR in rad26⌬ cells. We propose that the Spt5 CTR suppresses Rad26-independent TCR by serving as a platform for assembly of a multiple protein suppressor complex that is associated with Pol II. Phosphorylation of the Spt5 CTR by the Bur kinase may facilitate the assembly of the suppressor complex.
Nucleotide excision repair (NER)2 is a conserved DNA repair mechanism capable of removing a variety of helix-distorting lesions, such as UV-induced cyclobutane pyrimidine dimers (CPDs) (1). NER can be grouped into two pathways: global genomic repair (GGR), which refers to repair throughout the genome, and transcription coupled repair (TCR), which refers to a repair mechanism that is dedicated to the transcribed strand of actively transcribed genes (2). In the yeast Saccharomyces cerevisiae, Rad7, Rad16 (3), and Elc1 (4) are specifically required for GGR, but dispensable for TCR. Rad7 and Rad16 form a complex that binds specifically to UV-damaged DNA in an ATP-dependent manner and has DNA-dependent ATPase activity (5). Elc1 has been shown to be a component of a ubiquitin ligase that contains Rad7 and Rad16, and is responsible for regulating the levels of Rad4 protein in response to UV damage (6, 7). It has also been suggested that Elc1 is a component of another ubiquitin ligase complex, which contains Ela1, Cul3, and Roc1 but not Rad7 and Rad16 (8,9). The role of Elc1 in GGR may not be subsidiary to that of Rad7 and Rad16 (4).The mechanistic details of TCR are relatively well understood in Escherichia coli. The transcription repair coupling factor Mfd targets the transcribed strand for repair by recognizing a stalled RNA polymerase and actively recruiting the NER machinery to the transcription blocking lesion as it dissociates the stalled RNA polymerase (10). Conversely, the TCR mechanisms in eukaryotes appear to be extremely complicated (for reviews, see Refs. 11 and 12). In mammalian cells, Cockayne syndrome group A (CSA) and B (CSB) proteins are specifically required for TCR, but dispensable for GGR (13-16). Like its human homolog CSB, the yeast Rad26 plays an important role in TCR but is dispensable for GGR (17). Both human CSB (18) ...