Sex is determined in C. elegans by an X chromosome counting mechanism. The developmental switch gene
xol-1 assesses the number of X chromosomes by responding to the dose of a small number of X-linked regulatory genes called signal elements. XX animals have a double dose of these signal elements, allowing
xol-1 to be repressed and hermaphrodite development to ensue. XO animals have a single dose of X signal elements, allowing
xol-1 to be activated and male development to ensue. Two X signal elements have been extensively characterized: the nuclear hormone receptor SEX-1, which represses
xol-1 transcriptionally, and the RNA binding protein FOX-1, which represses
xol-1 post-transcriptionally. XOL-1 expression and activity are dependent on proper splicing of its sixth intron. Transgenic studies by M. Nicoll have shown that this sixth intron is both necessary and sufficient for FOX-1 regulation of
xol-1 expression. Increased doses of FOX-1 correlate with aberrant splicing of the sixth intron in vivo . We have begun investigating the detailed molecular mechanisms of FOX-1 activity. Preliminary data suggest that FOX-1 binds directly to the
xol-1 sixth intron. In addition to this detailed study of the known X signal element
fox-1 , we are characterizing a putative X signal element defined by the mutation
y323 . This mutation was found in a screen for suppressers of the total XO lethality caused by duplicating two regions on the left end of X known to contain other signal elements.
y323 is not located on the duplicated portion of X or in any of the other regions known to contain X signal elements. It has no phenotype on its own, but exhibits strong synergistic sex determination and dosage compensation defects in combination with
fox-1 . We have mapped
y323 to a 1.2 cM interval on the left half of X and are further characterizing this mutation in order to learn more about the regulation of
xol-1 activity by the X signal.