MEX-1, POS-1 and PIE-1 are required for germline development, share a CCCH-type "finger" domain of unknown biochemical function, and are expressed predominantly in germline blastomeres. A mutation in a new gene,
mex-5, has germ cells but has somatic defects that overlap with those of
mex-1. I cloned
mex-5, and found that it encodes a protein with the same CCCH "finger" domain as MEX-1, POS-1 and PIE-1. In addition, MEX-5 has a C. elegans homolog, AH6.5, that is about 70 percent identical at the protein level to MEX-5. A strain provided by Ron Plasterk which contains a deletion in the coding region of
ah6.5, including part of the conserved domain, has no phenotype. However, mothers mutant for both
mex-5 and
ah6.5 produce embryos which lack germ cells. The somatic phenotype of double mutant embryos also suggests that
mex-5 and
ah6.5 have redundant functions, since it is very different from the somatic phenotype of
mex-5 mutant embryos alone. Although I am still analyzing the basis for the germline defect in the double mutant, somatic defects include the misexpression of PIE-1 and other proteins. This misexpression may be due to ectopic translation, since misexpression occurs in blastomeres which in wild-type contain the RNA for these proteins.