Two Ustilago maydis viral dsRNAs of different size code for the same product.

UmV is a double-stranded RNA (dsRNA) virus of the corn fungus Ustilago maydis. There are three viral subtypes, P1, P4 and P6, which differ in the specificity of their secreted killer toxins. Each has three size classes of dsRNAs: H (heavy), M (medium), and L (light). We find that, unique among dsRNA viruses, two segments of different size code for the same product--the toxin resistance factor. The smaller dsRNA (L) is homologous to one end of the larger (M), and may have arisen by replication and packaging of a sub-genomic mRNA. We have also compared all the UmV dsRNAs with each other and with the dsRNAs of the similar yeast virus (ScV) by Northern gel and by 3' sequence analysis. Like those of ScV, many of the UmV dsRNAs have one 3' terminus with the sequence UUUUUCAOH or UUUUUCGOH. The H and L dsRNAs of similar size in different viral subtypes are generally related in sequence. The UmV H dsRNAs of different size are not detectably related in sequence.