The ciliate Euplotes octocarinatus expresses two polypeptide release factors of the type eRF1

Amplification of macronuclear DNA of the ciliate Euplotes octocarinatus rev ealed the presence of two genes encoding putative polypeptide release facto rs (RFs) of the codon specific class-I type. They are named eRF1a and eRF1b , respectively. cDNA amplification revealed that both eRF1 genes are expres sed. Determination of their copy numbers showed that they are similarly amp lified to a level of about 27,000. The deduced protein sequences of the two genes are 57 and 58% identical with human eRF1 and 79% identical to each o ther. The gene encoding eRF1b possesses three in-frame UGA codons. This cod on is known to encode cysteine in Euplotes: only UAA and UAG are used as st op codons in this organism. The primary structure of the two release factor s is analyzed and compared with the primary structure of other eukaryotic r elease factors including the one of Tetrahymena thermophila which uses only UGA as a stop codon. eRF1a and eRF1b of Euplotes as well as eRF1 of Tetrah ymena differ from human eRF1 and other class-I release factors of eukaryote s in a domain recently proposed to be responsible for codon recognition. Ba sed on the changes which we observe in this region and the differential use of the stop codons in these two ciliates we predict the amino acids partic ipating in stop codon recognition in eRF1 release factors. (C) 2001 Elsevie r Science B.V. All rights reserved.