Many pathways have been identified in eukaryotes that are responsible for the repair of damaged DNA. In humans, repair of UV-induced photolesions is dependent on the multistep nucleotide excision repair (NER) system. An absence of NER activity in humans results in the disorder xeroderma pigmentosum (XP). XP patients suffer from extreme photosensitivity, a high incidence of skin cancers and neurological defects. Most studies of NER have employed yeast or somatic cell lines; hence, we are interested in understanding how defects in NER affect the development and survival of a multicellular organism, such as C. elegans.
rad-3(
mn157) mutants are hypersensitive to UV because they are defective in NER [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 K07G5.2 deletion mutant from the KO consortium (
ok698). Both
rad-3 alleles are likely to be null because they encode truncated proteins, which are missing essential XPA domains. We have successfully rescued the UV sensitivity of both
rad-3 mutants using a
rad-3::gfp construct; RAD-3::GFP is expressed in nuclei throughout development, and in most tissues.UV irradiation of
rad-3 mutants leads to an arrest in growth and a decrease in survival. We show that the growth arrest in
rad-3 mutants after exposure to UV is due to an inhibition in cell division. In addition we find that the lifespan of
rad-3 mutants is reduced after UV treatment throughout all stages of development; however lifespan is most severely reduced during transcriptionally active stages, such as the late embryo and L1. By contrast, transcriptionally quiescent dauers remain healthy. We postulate that the death of
rad-3 mutants following UV exposure is due to the stalling of transcription complexes on unrepaired UV-lesions leading to the degradation of RNA pol II (AMA-1)[2]. We present three lines of evidence to support this model: 1) as discussed above,
rad-3 mutants display a heightened hypersensitivity to UV during transcriptionally active stages of development 2) the ability to initiate transcription is lost and 3) AMA-1 is preferentially degraded in
rad-3 mutants after UV exposure. We believe our results are the first to highlight the effects of UV damage on transcription and its consequences on the survival and development of a multicellular organism. [1] Hartman et al. Genetics 122, 379-85. (1989) [2] Ratner et al. J Bio Chem 273, 5184-89. (1998)