TwoUstilago maydisviral dsRNAs of differentsizecode for the same product

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Abstract
UmV is a double-stranded RNA (dsRNA) virus of the corn fungus Ustilago maydin. There are three viral subtypes, PI, P4 and P6, which differ in the specificity of their secreted killer toxins. Bach has three size classes of dsRNAs: H (heavy), M (medium), and L (light). We find that, unique among dsRNA viruses, two segments of diferent 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.