Genome Features of a New Double-Stranded RNA Helper Virus (LBCbarr) from Wine Torulaspora delbrueckii Killer Strains.

Manuel Ramírez,Alberto Martínez,Rocío Velázquez,Antonio López-Piñeiro

doi:10.3390/ijms222413492

Manuel Ramírez, Alberto Martínez + Show 2 more

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https://doi.org/10.3390/ijms222413492

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Abstract

The killer phenotype of Torulaspora delbrueckii (Td) and Saccharomyces cerevisiae (Sc) is encoded in the genome of medium-size dsRNA viruses (V-M). Killer strains also contain a helper large size (4.6 kb) dsRNA virus (V-LA) which is required for maintenance and replication of V-M. Another large-size (4.6 kb) dsRNA virus (V-LBC), without known helper activity to date, may join V-LA and V-M in the same yeast. T. delbrueckii Kbarr1 killer strain contains the killer virus Mbarr1 in addition to two L viruses, TdV-LAbarr1 and TdV-LBCbarr1. In contrast, the T. delbrueckii Kbarr2 killer strain contains two M killer viruses (Mbarr1 and M1) and a LBC virus (TdV-LBCbarr2), which has helper capability to maintain both M viruses. The genomes of TdV-LBCbarr1 and TdV-LBCbarr2 were characterized by high-throughput sequencing (HTS). Both RNA genomes share sequence identity and similar organization with their ScV-LBC counterparts. They contain all conserved motifs required for translation, packaging, and replication of viral RNA. Their Gag-Pol amino-acid sequences also contain the features required for cap-snatching and RNA polymerase activity. However, some of these motifs and features are similar to those of LA viruses, which may explain that at least TdV-LBCbarr2 has a helper ability to maintain M killer viruses. Newly sequenced ScV-LBC genomes contained the same motifs and features previously found in LBC viruses, with the same genome location and secondary structure. Sequence comparison showed that LBC viruses belong to two clusters related to each species of yeast. No evidence for associated co-evolution of specific LBC with specific M virus was found. The presence of the same M1 virus in S. cerevisiae and T. delbrueckii raises the possibility of cross-species transmission of M viruses.

Highlights

Killer yeast strains can kill non-killer strains because they secrete proteins that are killer toxins
T. delbrueckii Kbarr2 killer strain (EX1257) contains two M killer viruses, Mbarr1 and M1 [3], and we have recently discovered that this strain contains only a LBC virus (TdV-LBCbarr2) to maintain both M viruses
Two different sequences were obtained from the L dsRNA present in the Kbarr1 EX1180 strain, one similar to that of LA virus genomes that was previously characterized [12], and another sequence similar to that of LBC viruses

Summary

Introduction

Killer yeast strains can kill non-killer strains because they secrete proteins that are killer toxins. A certain type of killer yeast can kill other types of killer strains that belong to the same yeast species. Some killer yeast strains can kill yeasts (killer and non-killer) that belong to other species. Each killer yeast is immune to its own toxin and to toxins secreted by other yeast strains with the same type of killer phenotype [1,2,3]. Best known killer phenotype of Torulaspora delbrueckii (Td) and Saccharomyces cerevisiae (Sc) is encoded by cytoplasmic medium-size dsRNA viruses (V-M). The killer yeast strains contain a helper large-size (4.6 kb) dsRNA virus (V-LA) that is required for maintenance and replication of V-M. The helper V-LA provides the capsid and polymerase required for

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