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

Nieves Rodríguez-Cousiño,Rosa Esteban,Pilar Gómez

doi:10.3390/toxins9100313

Nieves Rodríguez-Cousiño, Rosa Esteban + Show 1 more

Open Access

https://doi.org/10.3390/toxins9100313

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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.

Highlights

Many yeasts produce killer toxins that kill other yeast of the same or different species
We have demonstrated the existence of different L-A variants in killer strains of S. cerevisiae, each one associated to a toxin-encoding M double-stranded RNA (dsRNA) satellite
We have characterized killer viruses from different Saccharomyces sensu stricto species, namely S. paradoxus, and clarified previous reports that wrongly assigned the production of certain killer toxins to S. cerevisiae and/or to S. paradoxus

Summary

Introduction

Many yeasts produce killer toxins that kill other yeast of the same or different species. Most killer toxins are encoded on double-stranded RNA (dsRNA) viruses called M, which depend for their maintenance on a helper virus (generically called L-A) that provides the viral capsids where L-A and. The so-called “killer system” has been extensively studied in S. cerevisiae [5,6] where four different killer toxins have been described: K1, K2, K28 and Klus, encoded on M1, M2, M28 and Mlus dsRNAs, respectively. Their sizes vary from 1.5 to 2.4 Kb [7,8,9,10]

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