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

Abstract

ABSTRACT 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.