. In eukaryotes, UV-induced photolesions are repaired through the multistep nucleotide excision repair (NER) pathway. In humans, the absence of NER activity results in the disorder xeroderma pigmentosum (XP). XP patients suffer from extreme photosensitivity, a high incidence of skin cancer and neurological defects. Most studies of NER have employed the single-celled yeast or somatic cell lines. To understand how defects in NER affect the development and survival of a multicellular organism, we have characterised a UV hypersensitive and NER defective mutant,
rad-3 [1].. We have established that the
rad-3 gene encodes the human XPA homolog (K07G5.2), a DNA-binding protein that is essential in the early steps of NER. We also obtained a deletion mutant from the KO consortium,
xpa-1(
ok698). Both alleles of
xpa-1 are likely to be null because they encode truncated proteins, which are missing essential domains. In the absence of UV exposure,
xpa-1 worms develop normally; we have shown that
xpa-1(
ok698) mutants have a low rate of spontaneous mutation and are not hypersensitive to reactive oxygen species (ROS). By contrast, when
xpa-1 animals are exposed to UV irradiation, they undergo an immediate growth arrest and decline in survival, and become hypermutable. Surprisingly, transcriptionally quiescent dauer stage larvae survive after receiving UV doses that are lethal to worms at other stages of development. We have successfully rescued the UV sensitivity of both
xpa-1 mutants using a XPA-1::GFP construct; XPA-1::GFP is expressed in nuclei throughout development, and in most tissues.. We further show that the UV-induced growth arrest and subsequent death of
xpa-1 worms is correlated with transcriptional inhibition. This inhibition is probably due to the stalling of transcription complexes on unrepaired UV-lesions, a phenomenon that leads to the proteolytic degradation of the large subunit of RNA Polymerase II [2]. In support of this hypothesis we show that the steady-state levels of Pol II decline in
xpa-1 worms after UV exposure. Finally, we demonstrate that the degradation of Pol II is dependent on the activity of an Rsp-5 like E3 ubiquitin ligase, encoded by the
wwp-1 gene. The UV sensitivity of the
wwp-1 xpa-1 double mutant is greater than either single mutant alone. These results, together with studies in yeast, establish that the ubiquitylation and degradation of Pol II aids the survival of organisms after they are exposed to UV irradiation. [1] Hartman et al. Genetics 122, 379-85. (1989), [2] Ratner et al. J Bio Chem 273, 5184-89. (1998).