Department of Biochemistry, Indian Institute of Science, Bangalore-500012.
Abstracts of Research Papers Presented during the National Symposium of Indian Virological Society at Unit of Plant Virology, Division of Plant Pathology, Indian Agricultural Research Institute, New Delhi-110 012, October 14–1.
Pepper vein banding virus (PVBV), a member of the genus potyvirus is a single-stranded positive-sense RNA virus which infects plants of the Solanaceae family. Genome organization and gene expression strategy of potyviruses are similar to that of picomaviruses, although they infect widely different hosts and have distinctly different morphologies. The complete genomic sequence of Pepper vein banding virus (PVBV), was determined and compared with other potyviruses. The viral genome contained 9711 nucleotides, excluding the poly-A tail. As in other potyviruses, the PVBV genome has a single open reading frame (ORF) starting at nucleotide 164 and ending at 9430, which encodes a polyprotein of 3088 amino acid residues. Phylogenetic analysis of the potyviral polyprotein sequences showed that PVBV is a distinct species of this genus.
The NIa proteinase from PVBV is a sequence specific proteinase required for processing of viral poly-protein in the cytoplasm. It accumulates in the nucleus of the infected plant cell and forms inclusion bodies. The function of this protein in the nucleus is not clear. The purified recombinant NIa proteinase was active and the mutation of the catalytic residues H46, D81 and C151 resulted in complete loss of activity. Interestingly the PVBV NIa proteinase exhibited hitherto unidentified activity, namely non-specific dsDNA degradation. This deoxyribonuclease (DNase) activity of the NIa proteinase showed an absolute requirement for Mg2+. Site-specific mutational analysis showed that of the three catalytic residues, D81 was the crucial residue for DNase activity. Based on the kinetic analysis and molecular modeling, a metal ion dependent catalysis similar to that observed in other non-specific DNases is proposed. Similar results were obtained with GST-fused PVBV NIa proteinase and Tobacco etch virus NIa proteinase, confirming that the DNase function is an intrinsic property of potyviral NIa proteinase. The NIa protein present in the infected plant nuclear extract also showed the proteinase and the DNase activities. Accumulation of this protein at high concentration in the nucleus occurs in the late stage of the infection, when viral RNA synthesis is almost complete in the cytoplasm. Therefore, host genomic DNA degradation and cell death could be a useful strategy for the virus in suppressing the host from mounting any systemic anti-viral defense.
