The phase II enzyme family of glutathione S-transferases (GSTs) is a major player in an organisms' defense against a variety of xenobiotics. To genetically dissect xenobiotic response pathways, we isolated abnormal GST-response mutations from
gst-4::gfp transgenic animals [1]. A dominant mutation,
xrep-3(
k1023), displays constitutively expressed GST-4::GFP throughout the whole body in the absence of xenobiotics. The
xrep-3 mutation also affects the expression of several other phase II enzymes such as GST-7, GST-30, GST-38, and C02D5.4. Conventional mapping linked
xrep-3 to LG IV, and SNP mapping narrowed it between the SNP marker T12E12 and
haw56365. We amplified a 16.9 kb DNA fragment containing T19E7.1, T19E7.2, and T19E7.3 from the
xrep-3 mutant genome, and injected it into
gst-4::gfp transgenic animals; this fragment induced GST-4::GFP expression without xenobiotics. The gene T19E7.2 encodes SKN-1, a nematode homolog of the bZIP transcription factor that controls metabolism, development, and stress responses. The gene
skn-1 is reported to encode the three protein isoforms SKN-1a, b, and c, which share the same C terminus with differing N termini. The mutation harbors a missense mutation in exon 3 of
skn-1a, which is the same as exon 1 of
skn-1c. We showed that
skn-1c cDNA from the
xrep-3 mutant constitutively induces GST-4::GFP expression.
Under non-stress conditions, the WD40 repeat protein XREP-1/WDR-23 physically interacts with SKN-1c and suppresses its activity together with CUL-4, DDB-1, and a proteasome [2]. By yeast two-hybrid experiments, we also confirmed that the
xrep-3 mutation suppresses the SKN-1c interaction with XREP-1/WDR-23. These results suggest that SKN-1c
(k1023) escapes from XREP-1/WDR-23 suppression, accumulates into cell nuclei, and constitutively induces GST expression in the absence of xenobiotics. We continue to uncover other genes executing pivotal roles in xenobiotic response pathways. [1] Hasegawa & Miwa, PLoS ONE (2010) 5:
e11194. [2] Choe et al., Mol. Cell. Biol. (2009) 29: 2704-15.