Ustilago Maydis Viruses

The viruses of Ustilago maydis are small, icosahedral, double-stranded RNA (dsRNA) viruses with undivided genomes. They are, like most other members of the family Totiviridae, normally innocuous and passed from cell to cell only by mating and cell division, although they are, inefficiently, infectious to protoplasts. Their genomic dsRNAs encode a single capsid polypeptide and an RNA-dependent RNA polymerase, at least in one case as a single fusion protein processed by endoproteolytic cleavage. Like most other dsRNA viruses, their particle structure is a T = 1 capsid composed of asymmetric dimers of identical subunits, a unique form of viral assembly. They have multiple satellite dsRNAs, some of which encode secreted polypeptide toxins (killer toxins) that may provide their host cells with a selective advantage. At least one of these toxins is derived from an ancestral nuclear gene and acts by interfering with calcium transport.