Anticodon nuclease encoding virus-like elements in yeast

A variety of yeast species are known to host systems of cytoplasmic linear dsDNA molecules that establish replication and transcription independent of the nucleus via self-encoded enzymes that are phylogenetically related to those encoded by true infective viruses. Such yeast virus-like elements (VLE) fall into two categories: autonomous VLEs encode all the essential functions for their inheritance, and additional, dependent VLEs, which may encode a toxin-antitoxin system, generally referred to as killer toxin and immunity. In the two cases studied in depth, killer toxin action relies on chitin binding and hydrophobic domains, together allowing a separate toxic subunit to sneak into the target cell. Mechanistically, the latter sabotages codon-anticodon interaction by endonucleolytic cleavage of specific tRNAs 3' of the wobble nucleotide. This primary action provokes a number of downstream effects, including DNA damage accumulation, which contribute to the cell-killing efficiency and highlight the importance of proper transcript decoding capacity for other cellular processes than translation itself. Since wobble uridine modifications are crucial for efficient anticodon nuclease (ACNase) action of yeast killer toxins, the latter are valuable tools for the characterization of a surprisingly complex network regulating the addition of wobble base modifications in tRNA.