Small Toxic Protein Encoded on Chromosome VII of Saccharomyces cerevisiae

In a previous study, we found an unknown element that caused growth inhibition after its copy number increased in the 3′ region of DIE2 in Saccharomyces cerevisiae. In this study, we further identified this element and observed that overexpression of a small protein (sORF2) of 57 amino acids encoded in this region caused growth inhibition. The transcriptional response and multicopy suppression of the growth inhibition caused by sORF2 overexpression suggest that sORF2 overexpression inhibits the ergosterol biosynthetic pathway. sORF2 was not required in the normal growth of S. cerevisiae, and not conserved in related yeast species including S. paradoxus. Thus, sORF2 (designated as OTO1) is an orphan ORF that determines the specificity of this species.