Control of the sperm-oocyte switch in Caenorhabditis elegans hermaphrodites by the fem-3 3' untranslated region.
 In the Caenorhabditis elegans hermaphrodite germ line, sperm and then oocytes are made from a common pool of germ-cell precursors.
 The decision to differentiate as a sperm or an oocyte is regulated by the sex-determining gene, fem-3.
 Expression of fem-3 in the hermaphrodite germ line directs spermatogenesis and must be negatively regulated to allow the switch to oogenesis.
 In adult hermaphrodites (which are producing oocytes), most fem-3 RNA is found in the germ line, consistent with both the requirement for fem-3 in hermaphrodite spermatogenesis and the maternal effects of fem-3 on embryonic sex determination.
 Whereas loss-of-function mutants in fem-3 produce only oocytes, hermaphrodites carrying any of nine fem-3 gain-of-function (gf) mutations make none; instead sperm are produced continuously and in vast excess over wild-type amounts.
 Genetic analyses suggest that fem-3(gf) mutations have escaped a negative control required for the switch to oogenesis.
 Here we report that all nine fem-3(gf) mutants carry sequence alterations in the fem-3 3' untranslated region (3' UTR).
 There is no increase in the steady-state level of fem-3(gf) RNA over wild-type, but there is an increase in the polyadenylation of fem-3(gf) RNA that is coincident with the unregulated fem-3 activity.
 Results of a titration experiment support the hypothesis that a regulatory factor may bind the fem-3 3' UTR.
 We speculate that fem-3 RNA is regulated through its 3' UTR by binding a factor that inhibits translation, and discuss the idea that this control may be part of a more general regulation of maternal RNAs.
