The inability to efficiently repair or remove cells with damaged genomes has been linked to cancer development. DNA damage induces multiple biological responses that insure the integrity of the genome. Checkpoint genes that sense the damaged DNA and transduce signals to the transcription, cell cycle, repair and apoptotic machinery mediate these responses. In C. elegans, DNA damage induces cell cycle arrest and apoptosis in the mitotic and meoitic germ cells, respectively. The core apoptotic machinery and the
p53 homolog,
cep-1, are required for proper activation of DNA damage induced apoptosis. Furthermore, mutations in several genes have identified a conserved checkpoint pathway that mediates germ cell cycle arrest and apoptosis in response to genotoxic stress(1). These include
hus-1,
mrt-2, and
clk-2/rad-5. In order to identify new genes involved in DNA damage responses in C. elegans, we employed a genetic screen for mutants defective in ionizing radiation induced cell cycle arrest. In an ongoing screen, we have identified one mutant,
op259, which displays defects for DNA damage responses, including cell cycle arrest and DNA damage induced apoptosis.
op259 mutants display embryonic let hality, slow growth and show abnormally small germ cell nucleoli. Using a combination of SNP mapping and RNAi, we have determined that the affected gene is the beta subunit of RNA Pol I.
op259 is a hypomorph with a mutation resulting in a P70S substitution, a residue well conserved with Pol I in other species and with C. elegans RNA Pol II. Recently, the transcription factor complex TFIIH has been shown to be required for RNA Pol I transcription(2). Furthermore, the TFIIH subunits XPD and XPB have been demonstrated to be required for
p53 activity and for transcription mediated repair(3). Associations of TFIIH with components of the transcription, repair, cell cycle and apoptotic machinery could play a key role in deciding between life and death following DNA damage. It is possible that the
op259 mutation in RNA Pol I alters TFIIH distribution in the cell, although other mechanisms may exist.1.A. Gartner, et al. Mol Cell 5, 435-43. (2000); 2.S. Iben et al., Cell 109, 297-306. (2002); 3.X. W. Wang et al., Genes Dev 10, 1219-32. (1996).