Cassava Brown Streak Virus ( Potyviridae ) Encodes a Putative Maf/HAM1 Pyrophosphatase Implicated in Reduction of Mutations and a P1 Proteinase That Suppresses RNA Silencing but Contains No HC-Pro

The complete positive-sense single-stranded RNA genome of Cassava brown streak virus (CBSV; genus Ipomovirus; Potyviridae) was found to consist of 9,069 nucleotides and predicted to produce a polyprotein of 2,902 amino acids. It was lacking helper-component proteinase but contained a single P1 serine proteinase that strongly suppressed RNA silencing. Besides the exceptional structure of the 5'-proximal part of the genome, CBSV also contained a Maf/HAM1-like sequence (678 nucleotides, 226 amino acids) recombined between the replicase and coat protein domains in the 3'-proximal part of the genome, which is highly conserved in Potyviridae. HAM1 was flanked by consensus proteolytic cleavage sites for ipomovirus NIaPro cysteine proteinase. Homology of CBSV HAM1 with cellular Maf/HAM1 pyrophosphatases suggests that it may intercept noncanonical nucleoside triphosphates to reduce mutagenesis of viral RNA.