The Papaya ringspot virus (PRSV) is one of the limiting factors affecting papaya and cucurbits production worldwide. PRSV belongs to the potyvirus genus which consists of 30% of known plant viruses. Two serological closely related strains, namely type-P and -W, have been reported. PRSV type-P infects both papaya and cucurbits, while type-W infects only cucurbits. The genome of PRSV Thailand isolate consists of a (+)
RNA molecule of 10323
nucleotides, which is first translated into a single
polypeptide and further cleaved by three viral encoded
proteases into ten gene products. Helper-component
proteinase (
HC-Pro), which is encoded by the 2nd cistron of the potyviral genome, has been implicated in aphid transmission, viral movement, viral replication and suppression of host viral defense system. Studies of the Tobacco etch virus (TEV), Lettuce mosaic virus (LMV), Onion yellow dwarf virus (OYDV) and Wheat streak mosaic virus (WSMV) indicate that the N-terminal of
HC-Pro is dispensable for systemic
infection in their respective hosts. However, deletion analysis of the Tobacco vein mottling virus (TVMV) indicates otherwise. In this study, we examined whether
HC-Pro is essential for PRSV systemic
infection in cucurbits and the role of its N-terminal in systemic
infection. Our results indicated that
HC-Pro is indispensable for PRSV
infection in zucchini. Deletion analysis of PRSV
HC-Pro showed that deletion of as few as 54
amino acids at the N-terminal of
HC-Pro completely abolished the infectivity of the corresponding
cDNA clone. Therefore, it is proposed that the N-terminal of
HC-Pro is involved in systemic
infection of PRSV, in addition to its conserved function in aphid transmission.