Variation and Distribution of L-A Helper Totiviruses in Saccharomyces sensu stricto Yeasts Producing Different Killer Toxins

Abstract

Yeasts within the Saccharomyces sensu stricto cluster can produce different killer toxins. Each toxin is encoded by a medium size (1.5–2.4 Kb) M dsRNA virus, maintained by a larger helper virus generally called L-A (4.6 Kb). Different types of L-A are found associated to specific Ms: L-A in K1 strains and L-A-2 in K2 strains. Here, we extend the analysis of L-A helper viruses to yeasts other than S. cerevisiae, namely S. paradoxus, S. uvarum and S. kudriavzevii. Our sequencing data from nine new L-A variants confirm the specific association of each toxin-producing M and its helper virus, suggesting co-evolution. Their nucleotide sequences vary from 10% to 30% and the variation seems to depend on the geographical location of the hosts, suggesting cross-species transmission between species in the same habitat. Finally, we transferred by genetic methods different killer viruses from S. paradoxus into S. cerevisiae or viruses from S. cerevisiae into S. uvarum or S. kudriavzevii. In the foster hosts, we observed no impairment for their stable transmission and maintenance, indicating that the requirements for virus amplification in these species are essentially the same. We also characterized new killer toxins from S. paradoxus and constructed “superkiller” strains expressing them.

Saccharomyces paradoxus K66 Killer System Evidences Expanded Assortment of Helper and Satellite Viruses

“…Double-stranded RNA-based killer systems have been described in different yeast species, such as Saccharomyces cerevisiae , S. paradoxus , S. uvarum , Ustilago maydis , Zygosaccharomyces bailii , Hanseniaspora uvarum , and Torulaspora delbrueckii [ 5 , 6 , 7 ]. The mycoviruses of the Totiviridae family are incapsulated into virus-like particles (VLPs) and stably persist in the host cell without causing cell lysis; they are transmitted by vegetative cell division or through sexual fusion [ 5 , 8 , 9 , 10 ].…”

“…The M dsRNA viruses show no sequence homology to each other, though the organization of their genomes is strikingly similar. The positive strand contains an open reading frame (ORF) in the 5‘-terminal region that encodes for the toxin precursor, followed by a unique internal AU-rich region, and a 3‘-terminal non-coding region of variable length possessing cis signals for encapsidation and replication by the viral RNA polymerase [ 6 ]. Different killer toxins are secreted glycoproteins lacking amino acid sequence conservation and adopting diverse cell killing mechanisms.…”

“…Saccharomyces paradoxus is the closest relative of the domesticated yeast S. cerevisiae , mainly found in the wild [ 6 ]. Killer toxins of S. paradoxus have been considered as chromosome-coded for a long time and it is only relatively recently that dsRNA viruses (L and M) in such yeast have been discovered [ 44 ].…”

Adaptive evolution of nontransitive fitness in yeast

“…Though there are several different killer viruses in yeast (e.g. K1, K2, K28, Klus), each arose independently and has a distinct mechanism of action (Rodríguez-Cousiño et al 2017). Nontransitive interactions may therefore arise frequently through cycles of gains and losses of toxin production and toxin immunity in lineages that contain RNA viruses.…”

Saccharomyces cerevisiae killer toxins: synthesis, mechanisms of action and practical use

“…Особый интерес вызывает коэволюция вирусов L-A и М. При изучении нуклеотидных последовательностей днРНК L-А было выявлено по крайней мере четыре природных варианта днРНК L-A. Их анализ продемонстрировал, что они отличаются между собой примерно на 24 % [58]. При этом авторы показали, что подбор эффективных пар вирус-помощник и вирус-киллер произошел путем коэволюции и пары вирусов специфически адаптированы друг к другу.…”