Glutathione S-transferases (GSTs) are evolutionary ancient enzymes that detoxify xenobiotics and promote cellular redox balance. The C. elegans genome encodes over 30 GSTs, many of which are transcriptionally induced by xenobiotics and oxidative stress. Using a transgenic reporter, we previously performed a genome-wide RNAi screen for regulators of
gst-4 expression. The WD40 repeat protein WDR-23 was the strongest repressor of
gst-4 and we reported that it functions with the CUL-4/DDB-1 ubiquitin ligase to repress nuclear accumulation and activation of the transcription factor SKN-1. We have now begun to characterize some of the other 9 repressors and 12 activators of
gst-4 that were identified with our RNAi screen. RNAi for most of the other repressors does not have an additive effect on
gst-4 expression when combined with a null allele of
wdr-23 suggesting that they function in common pathways. One exception is F28D1.1, a second WD40 repeat family member. F28D1.1 is an orthologue of the yeast protein UTP7, which functions in ribosome biogenesis, kinetochore organization, and chromosome segregation. Almost nothing is known about F28D1.1 or its orthologues in animals. Real-time RT-PCR data from a deletion mutant confirm that F28D1.1 represses mRNA levels of endogenous
gst-4 and
gst-30 by a mechanism that involves
skn-1 but not
daf-16. Unlike WDR-23, F28D1.1 protein does not interact with SKN-1 in the yeast 2-hybrid system suggesting that regulation is indirect. GFP reporter analysis suggests that F28D1.1 protein is expressed in the nucleolus of many cells and F28D1.1 loss-of-function increases resistance to the xenobiotic juglone. Multiple C. elegans nucleolar proteins are predicted to interact with F28D1.1 based on interactions between homologous yeast proteins (STRING search). Preliminary data indicate that loss-of-function for most of these nucleolar proteins increases
gst-4 expression similar to F28D1.1(RNAi). Taken together, our data suggest that disruption of nucleolar function activates stress-resistance genes via SKN-1. This novel mechanism of SKN-1 regulation appears to function separately from WDR-23.