RAD25 IS A DNA HELICASE REQUIRED FOR RNA REPAIR AND RNA-POLYMERASE-IITRANSCRIPTION

THE RAD25 gene of Saccharomyces cerevisiae functions in nucleotide exc ision repair of ultraviolet-damaged DNA and is also required for cell viability(1). The RAD25 protein shows remarkable homology to the prote in encoded by the human nucleotide-excision-repair gene XPB (ERCC3), m utations in which cause the cancer-prone disease xeroderma pigmentosum and also Cockayne's syndrome(1). Here we purify RAD25 protein from S. cerevisiae and show that it contains single-stranded DNA-dependent AT Pase and DNA helicase activities. Extract from the conditional lethal mutant rad25-ts(24) exhibits a thermolabile transcriptional defect whi ch can be corrected by the addition of RAD25 protein, indicating a dir ect and essential role of RAD25 in RNA polymerase II transcription. Th e protein encoded by the rad25(799am) allele is defective in DNA repai r but is proficient in RNA polymerase II transcription, indicating tha t RAD25 DNA-repair activity is separable from its transcription functi on. The rad25 Arg-392 encoded product, which contains a mutation in th e ATP-binding motif, is defective in RNA polymerase II transcription, suggesting that the RAD25-encoded DNA helicase functions in DNA duplex opening during transcription initiation.