Protein Splicing of Yeast TFP1: Evidence for a New Class of Mobile Genetic Elements

Summary

The TFP1 gene of the yeast S. cerevisiae has been found to encode two proteins, the 69 kDa subunit of the vacuolar H⁺-ATPase, and a 50 kDa ‘spacer’ protein. The 50 kDa spacer protein is encoded by the central 454 codons of TFP1, whereas the first 283 and last 334 amino acids are post-translationally connected by a peptide bond to form the 69 kDa polypeptide. Because the reaction involves excision of an intervening protein domain and the religation of the N- and C-terminal regions of the Tfpl protein, the process was named protein splicing. The Tfpl precursor has been found to undergo efficient protein splicing even when expressed in E. coli, suggesting that this reaction occurs by a self-splicing mechanism. The spacer protein has been found to undergo protein splicing independent of its insertional context, indicating that all the information necessary for protein splicing is contained within the spacer protein. The Tfpl spacer protein is an endonuclease that specifically cleaves the TFP1DNA lacking the intervening sequence, leading to gene conversion of the allele lacking the intervening sequence by the wild-type TFP1 allele. These data indicate that the TFP1 spacer DNA defines a new class of mobile genetic elements, which move at the DNA level and excise by protein splicing to remain phenotypically silent.