In species with variant genetic codes, one or two stop codons encode amino acid residues and are

not recognized by the intrinsic class I translation termination factor (eRF1). Ciliata include a large number of species with variant genetic codes. The stop codon specificity of the Blepharisma japonicum translation termination factor eRF1 was determined in an in vitro eukaryotic translation system and in an in vivo assay (a dual reporter system). It was shown that eRF1 of B. japonicum retained specificity to all three stop codons, although the efficiency of peptydyl-tRNA hydrolysis in the presence of UGA was reduced in the in vitro assay. Since Heterotrichea (including B. japonicum) are the earliest diverged lineage in the phylogenetic tree of ciliates, B. japonicum probably possesses a universal genetic code similar to the putative ciliate ancestor group.