PIE-1 is a bifunctional protein that regulates maternal and zygotic gene expression in the embryonic germ line of Caenorhabditis elegans

The CCCH zinc finger protein PIE-1 is an essential regulator of germ cell f ate that segregates with the germ lineage during the first cleavages of the Caenorhabditis elegans embryo. We have shown previously that one function of PIE-1 is to inhibit mRNA transcription. Here we show that PIE-1 has a se cond function in germ cells; it is required for efficient expression of the maternally encoded Nanos homolog NOS-2. This second function is geneticall y separable from PIE-1's inhibitory effect on transcription. A mutation in PIE-1's second CCCH finger reduces NOS-2 expression without affecting trans criptional repression and causes primordial germ cells to stray away from t he somatic gonad, occasionally exiting the embryo entirely. Our results ind icate that PIE-1 promotes germ cell fate by two independent mechanisms as f ollows: (1) inhibition of transcription, which blocks zygotic programs that drive somatic development, and (2) activation of protein expression from n os-2 and possibly other maternal RNAs, which promotes primordial germ cell development.