''Saccharomyces cerevisiae virus L-A'', also called L-A helper virus, is a member of the ''

Totiviridae ''Totiviridae'' is a family of double-stranded RNA viruses. Giardia lamblia, leishmania, trichomonas vaginalis, and fungi serve as natural hosts. The name of the group derives from Latin ''toti'' which means undivided or whole. There are 28 spec ...

'' family of viruses found primarily in ''

Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have been o ...

''. Its discovery in the 1970s was the main starting point of research on yeast virology. It is a ~4.6 kb

double-stranded RNA virus Double-stranded RNA viruses (dsRNA viruses) are a polyphyletic group of viruses that have double-stranded genomes made of ribonucleic acid. The double-stranded genome is used to transcribe a positive-strand RNA by the viral RNA-dependent RNA ...

with no extracellular phase and so is inherited through vertical cytoplasmic transmission. Additionally, in many strains of the yeast, it is found along with another virus called the M virus (or "M dsRNA", "Saccharomyces cerevisiae killer virus"), known to encode the killer toxin in many ''S. cerevisiae'' strains which confers the ability to kill neighboring sensitive cells that do not harbor the virus. It is indeed for this reason the virus is referred to as a

helper virus A helper virus is a virus that allows an otherwise-deficient coinfecting virus to replicate. These can be naturally occurring as with Hepatitis D virus, which requires Hepatitis B virus to coinfect cells in order to replicate. Helper viruses are a ...

, due to the M

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...

's dependence on it for its own survival and replication. (More recently, the M dsRNA is described as a

satellite RNA A satellite is a subviral agent that depends on the coinfection of a host cell with a helper virus for its replication. Satellites can be divided into two major classes: satellite viruses and satellite nucleic acids. Satellite viruses, which a ...

.) A Yeast lineage with a M/killer virus is known as a " killer strain". There are numerous apparently unrelated M dsRNAs that use L-A, their only similarity being their genome organization. The family of ''

'' in general helps M-type dsRNAs in a wide variety of yeasts.

References

{{taxonbar, from=Q7396598 Totiviridae Mycoviruses