We have previously shown that worm
p53 (
cep-1) is required for resistance to multiple stresses and promotes DNA damage-induced apoptosis in the germline (Derry et al., 2001, Science, Vol. 294, 591). Although earlier studies with a weaker allele did not reveal a role for
cep-1 in cycle cycle arrest, we found, using a strong knockout allele recently isolated by the C. elegans Knockout Consortium, that
cep-1 is required to promote both apoptosis and cell cycle arrest in the germline in response to UV radiation. To understand how
cep-1 modulates response to UV radiation, we have used microarrays to identify genes regulated by
cep-1. We compared UV-responsive genes in the strong loss-of-function mutant,
cep-1(
gk138), with wild-type in young adults exposed to UV. Expression of 17886 genes (94% of all annotated genes) was surveyed on microarrays. From this data set, over 500 genes were upregulated greater than 2-fold in a
cep-1-dependent manner by UV. These upregulated genes represent a wide variety of cellular functions that range from cell cycle control to collagen synthesis, indicating that a complex network of genes regulated by
cep-1 is involved in mounting a response to UV. To validate these target genes, we have been analyzing the genome for
p53 binding sites and have identified candidate cis-regulatory elements of several genes upregulated in the microarray experiments. One gene upregulated 3-fold by UV in a
cep-1-dependent manner is
phg-1, a homologue of the human growth arrest specific gene,
gas1. Identified as a gene that is over-expressed in arrested murine cells,
gas1 encodes a glycoprotein associated with the external plasma membrane by a GPI-anchor (Mullor & Altaba, 2002, BioEssays, Vol. 24, 22). Overexpression of
gas1 in mammalian cells causes growth arrest that is
p53 dependent; however, the mechanism by which
p53 and
gas1 cooperate remains unknown. Overexpression of
phg-1 in mammalian cells also causes growth arrest but expression in developing C. elegans embryos has no detectable phenotype (Agostoni et al., 2002, BBA, Vol. 1574, 1). We found a putative
p53 consensus binding site in the first intron of the
phg-1 gene, supporting our hypothesis that it is a direct transcriptional target of CEP-1. Using RNAi we have discovered that
phg-1 is required to arrest proliferation of germ cells in response to DNA damage. Unlike
cep-1, which has roles in both cell cycle control and apoptosis,
phg-1 does not affect germline apoptosis, consistent with its action downstream of
cep-1 and establishing a split in the pathway controlling DNA damage response in the germline.