pos-1 encodes a cytoplasmic zinc-finger protein essential for germline specification in C-elegans

Germ cells arise during early C. elegans embryogenesis from an invariant se quence of asymmetric divisions that separate germ cell precursors from soma tic precursors. We show that maternal-effect lethal mutations in the gene p os-1 cause germ cell precursors to inappropriately adopt somatic cell fates . During early embryogenesis, pos-1 mRNA and POS-1 protein are present pred ominantly in the germ precursors. POS-1 is a novel protein with two copies of a CCCH finger motif previously described in the germline proteins PIE-1 and MEX-1 in C, elegans, and in the mammalian TIS11/Nup475/TTP protein, How ever, mutations in pos-1 cause several defects in the development of the ge rmline blastomeres that are distinct from those caused by mutations in pie- 1 or mex-1. The earliest defect detected in pos-1 mutants is the failure to express APX-1 protein from maternally provided apx-1 mRNA, suggesting that POS-1 may have an important role in regulating the expression of maternal mRNAs in germline blastomeres.