Maternal mRNA from clam oocytes can be specifically unmasked in vitro by antisense RNA complementary to the 3'-untranslated region.

Clam oocytes display a striking difference in the pattern of protein synthesis on fertilization, which is maintained when cell-free extracts are assayed in the rabbit reticulocyte lysate. The mRNAs encoding ribonucleotide reductase and cyclin A (the major masked messages in the oocyte extracts) can be translationally activated by gel filtration in 0.5 M KCl, presumably by removal of repressor protein(s). When synthetic RNAs corresponding to different segments of ribonucleotide reductase and cyclin A mRNAs were added to oocyte extracts in 0.5 M KCl in a "competitive unmasking" assay, specific and complete unmasking of these mRNAs occurred on addition of antisense 3'-noncoding transcripts. The unmasking region in ribonucleotide reductase mRNA maps to a region of 134 nucleotides centered one-third of the way down the 3'-noncoding region.