LETHALITY IN YEAST OF TRICHOTHIODYSTROPHY (TTD) MUTATIONS IN THE HUMAN XERODERMA-PIGMENTOSUM GROUP-D GENE - IMPLICATIONS FOR TRANSCRIPTIONAL DEFECT IN TTD

Mutations in the human XPD gene result in a defect in nucleotide excis ion repair of ultraviolet damaged DNA and cause the cancer-prone syndr ome xeroderma pigmentosum (XP). Resides XP, mutations in XPD can cause another seemingly unrelated syndrome, trichothiodystrophy (TTD), char acterized by sulfur-deficient brittle hair, ichthyosis, and physical a nd mental retardation. To ascertain the underlying defect responsible for TTD, we have expressed the TTD mutant proteins in the yeast Saccha romyces cerevisiae and determined if these mutations can rescue the in viability of a rad3 null mutation. RAD3, the S. cerevisiae counterpart of XPD, is required for nucleotide excision repair and also has an es sential role in RNA polymerase II transcription. Expression of the wil d type XPD protein or the XPD Arg-48 protein carrying a mutation in th e DNA helicase domain restores viability to the rad3 null mutation. In terestingly, the XPD variants containing TTD mutations fail to complem ent the lethality of the rad3 null mutation, strongly suggesting that TTD mutations impair the ability of XPD protein to function normally i n RNA polymerase II transcription. From our studies, we conclude that XPD DNA helicase activity is not essential for transcription and infer that TTD mutations in XPD result in a defect in transcription.