In wild-type animals, strong Pdbl-1::gfp expression was observed in the M4 cell in 96% of transgenic adults. In comparison, strong Pdbl-1::gfp expression in M4 was observed in only 17% of ceh-28(cu11) adults (n=53)and 19% of ceh-28(tm1258) adults.
The M4 enhancer contains two candidate CEH-28 binding sites: one located at bp -825, which is conserved in other Caenorhabditis species, and a second at bp -738. Authors tested the effect of mutations in each of these sites on M4 enhancer activity either alone or in a double mutant. Mutation of the site at -825 led to a decrease in the intensity of GFP expression level and a moderate decrease in the frequency of animals expressing GFP, whereas mutation of the site at -738 had little effect on enhancer activity. In comparison, a double mutant affecting both of these sites had a stronger reduction in both the intensity and frequency of GFP expression in M4,