The
fog-1 and
fog-3 genes are required to specify that germ cells differentiate as sperm. One possible model is that
fog-1 and
fog-3 act in response to the sex-determination genes to regulate germ cell fates. Although many regulatory interactions in the germ line occur at the translational level,
fog-3 appears to act at the nexus of a set of transcriptional controls. 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. We used quantitative RT-PCR to show that
tra-1 acts downstream of the fem genes to regulate
fog-3 . We suspect that
tra-1 controls transcription of
fog-3 , rather than stability of the messages, since the levels of partially processed
fog-3 transcripts are also affected by mutations in the sex-determination genes. The TRA-1A binding site most proximal to
fog-3 might overlap sequences required for the primary
fog-3 transcript; this interference could explain in part how TRA-1A represses
fog-3 . Mutations in the fem genes have a weak effect on expression of
fog-3 in a
tra-1(
e1099) mutant background; thus we suspect that the fem genes not only inhibit TRA-1A, but also promote expression of
fog-3 in a separate manner. In addition,
tra-1; fem double mutants that produce wild-type levels of
fog-3 transcripts nonetheless make only oocytes, which indicates that the fem genes regulate another activity necessary for spermatogenesis. 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.