apg15-1, a UGA mutant allele in the Saccharomyces cerevisiae APG16 gene, and its suppression by a cytoplasmic factor.

Autophagy is a complex cellular process by which starving cells utilize cytoplasmic macromolecules as nutritional resources. In Saccharomyces cerevisiae, more than 15 genes are involved in this process and most of them have been cloned and characterized by now. But there remains a complementation group represented by a single mutation, apg15-1, unclear as to its molecular nature. We obtained DNA fragments that functionally complemented apg15-1 and found that the responsible ORF, YMR159C, was already assigned as APG16. It was further found that apg15-1 was a UGA allele in which the 243rd base of the 450 bp coding region of APG16 was converted from C to T, and that the previously observed complementation between apg15-1 and apg16D was attributable to the action of a cytoplasmic omnipotent suppressor. This suppressor was readily cured by guanidine-HCl and also by overexpression or disruption of HSP104, indicating its close similarity to the PSI prion-like factor. Since apg15-1 is a mutation highly sensitive to termination suppression, it can be used as a tool to detect weak termination suppressors.