Molecular characterization of Potato mop-top virus isolates from China and Canada and development of RT-PCR differentiation of two sequence variant groups

The complete genome comprising three genomic RNAs of three Canadian and two Chinese isolates of Potato mop-top virus were sequenced and analysed. Two open reading frames (ORFs) were found in RNA1 of 6.1 kb, encoding a readthrough RNA-dependent RNA polymerase (RdRp). A coat protein (CP)-readthrough protein was encoded by RNA2 of 3.1 kb. Four ORFs that encoded the triple gene block proteins (TGBps) and a cysteine-rich protein were found in RNA3 (2.9 kb) of the Chinese isolate 'Yunnan'; whereas in the remaining isolates (three Canadian isolates and the Chinese isolate 'Guangdong'), only three ORFs encoding TGBps were observed in RNA3. A single nucleotide mutation of A(2462) to G(2462) abolished the start codon 'AUG' for the fourth putative ORF in RNA3 of these isolates. Based on phylogenetic and sequence similarity analysis of these isolates as well as those reported by others at the complete RNA sequence level, each of RNA1, RNA2 and RNA3 could be divided into at least two groups. In Canadian isolates 'Ch9', 'Ch10' and 'Ch20' and Chinese isolate 'Guangdong', all genomic RNAs belonged to group A; and in Chinese isolate 'Yunnan', all of its RNA belonged to group B. Interestingly, in Swedish isolate 'Sw', RNA1 and RNA2 belonged to group A while RNA3 belonged to group B. A duplex RT-PCR for differentiating groups A and B of RNA3 was developed and evaluated. All PMTV samples collected in Guangdong, China, and New Brunswick, Canada, possessed a RNA3 belonging to group A; whereas the samples collected in Yunnan, China, possessed a RNA3 belonging to group B.