Identification of a developmentally regulated translation elongation factor 2 in Tetrahymena thermophila.

Protein synthesis elongation factor 2 (eEF2) catalyzes the translocation of the peptidyl-tRNA from the A site to the P site of the ribosome. Most organisms encode a single EF2 protein and its activity is regulated by phosphorylation. We have identified a family of genes in Tetrahymena thermophila that encode proteins homologous to eEF2, yet are expressed only during sexual reproduction. These genes have been designated EFR for Elongation Factor 2 Related. EFR transcripts were not detected in vegetative cell cultures but rapidly increased about 6 h after the start of conjugation (mating). For comparison, we cloned, sequenced and analyzed the expression of the standard eEF2 gene from T. thermophila. Unlike EFR, transcripts from eEF2 were detected in vegetative cells but were present at lower concentrations during conjugation. Despite the high sequence identity between EFR and eEF2 from other organisms (about 42% at the amino acid level), key regulatory sequences that are involved in the regulation of eEF2 are altered in EFR. The sequence and expression data suggest that EFR is an eEF2 variant involved in a major translation regulatory mechanism that occurs during the formation of the macronuclear genome in conjugating cells.