The first division of the nematode zygote creates two blastomeres, called AB and P1, that differ in their autonomous ability to produce muscle; P1 can produce muscle cells when cultured in isolation but AB cannot. We have previously reported that in
mex-3 mutant embryos, both AB and P1 have the autonomous ability to produce muscle cells. In contrast to
mex-1 and
par-1 mutant embryos, the AB blastomere in
mex-3 mutants does not require
skn-1(+) activity to produce muscle cells. Our results thus suggest that a factor other than SKN-1 is inappropriately expressed in the AB blastomeres in
mex-3 mutants and that the expression of this factor leads to muscle cell development. We have cloned the
mex-3 gene and shown that it encodes a novel, but highly conserved, cytoplasmic protein with two potential RNA-binding motifs called KH domains. We have sequenced three
mex-3 alleles that result in mutant phenotypes indistinguishable from null alleles yet produce the wild-type amount of protein. We have found that these mutations are changes in highly conserved residues in the second KH domain, indicating that the KH domains are required for
mex-3(+) activity. We find that
mex-3 mRNA accumulates predominately at the anterior pole of the zygote following fertilization and that MEX-3 protein is abundant in oocytes and early embryos. We propose that
mex-3(+) activity contributes to the very different muscle-forming abilities of the early blastomeres by regulating the expression of maternal mRNA(s).