The
fog-1 and
fog-3 genes are required to specify that germ cells differentiate as sperm. Although mutations in either gene cause germ cells to develop as oocytes instead, they have no effect on somatic fates. Thus, one simple model is that
fog-1 and
fog-3 act in response to the sex-determination genes to regulate germ cell fates. A region of less than 800 nucleotides is required to drive expression of
fog-3 on extra-chromosomal arrays. This promotor contains five potential TRA-1A binding sites, more than are found near any other C. elegans gene. Furthermore, this region of the
fog-3 promotor binds TRA-1A in gel-shift assays, and this binding is disrupted by mutations in these five sites. In addition, a mutation in the third site abolishes activity of
fog-3 transgenes, so we suspect that this site might help activate transcription of
fog-3. By contrast, a mutation in the fifth site does not lower activity of the transgenes. Quantitative RT-PCR results show that
tra-1 is needed to repress transcription of
fog-3 in L4 hermaphrodites. Since this fifth TRA-1A binding site appears to overlap sequences required for the primary
fog-3 transcript, this interference could explain how TRA-1A represses
fog-3. The amino-terminal 116 amino acids of FOG-3 are similar to the Tob, BTG1 and BTG2 proteins of vertebrates. We find that 6 out of 8 missense mutations in FOG-3 map to this domain, and several of these alter conserved residues. In vertebrates, this domain of BTG1 and BTG2 mediates binding to CAF-1, which is part of a transcriptional regulatory complex. RNA-mediated inactivation shows that
caf-1 is essential for both embryogenesis and germ-line function in C. elegans. We are now testing FOG-3 for direct interactions with CAF-1 and other regulatory proteins. To learn if the role of FOG-3 in germ cell fate has been conserved during evolution, we cloned the homologs of
fog-3 from two related nematode species C. briggsae, which is also male/hermaphroditic, and C. remanei, which is male/female. Although these species have different mating systems, the FOG-3 proteins are highly conserved. Furthermore, FOG-3 is required in each species for germ cells to become sperm rather than oocytes, but appears to play no other role in development. Finally, the expression of
fog-3 is correlated with spermatogenesis during the development of C. briggsae and C. remanei, just as with C. elegans. Since each of the
fog-3 promotors contain potential TRA-1A binding sites near the transcriptional start site, TRA-1A might control expression of
fog-3 in each species. These results suggest that changes in the transcriptional control of
fog-3 might explain the difference between female nematodes, which cannot produce sperm, and hermaphrodite nematodes, which can. We do not yet know what upstream genes have been altered to bring about these changes in
fog-3 expression.