DOMINANT FEMINIZING MUTATIONS IMPLICATE PROTEIN-PROTEIN INTERACTIONS AS THE MAIN MODE OF REGULATION OF THE NEMATODE SEX-DETERMINING GENE TRA-1

The tra-1 gene is the terminal global selector of somatic sex in Caeno rhabditis elegans: High tra-1 activity elicits female somatic developm ent while low tra-1 activity elicits male development. Previous geneti c studies defined a cascade of negatively interacting genes that regul ates tra-1 activity in response to the primary sex-determining signal. Here, we investigate the last step in this regulatory cascade, by stu dying rare gain-of-function (gf) mutations of tra-1 that direct female somatic development irrespective of the upstream sex-determining sign al. These mutations appear to abolish negative regulation of tra-1 in male tissues. We identify the lesions associated with 29 of these muta tions and find that all affect a short stretch of amino acid residues present in both protein products of the tra-1 gene. Twenty-six alleles are associated with single nonconservative amino acid substitutions. Two alleles affect tra-1 RNA splicing and generate messages that omit part or all of the exon encoding this short stretch. These results sug gest that sexual regulation of tra-1 is achieved post-translationally, by an inhibitory protein-protein interaction. The amino acid stretch altered by the tra-1(gf) mutations may define a site of interaction fo r negative regulators of tra-1. The stretch includes a potential phosp horylation site for glycogen synthase kinase 3 and may be conserved in the human gene GLI3, a homolog of tra-1 identified previously.