Mechanistic insights into the replication of Sesbania mosaic virus

Abstract

The present thesis deals with the mechanism of replication of Sesbania mosaic virus (SeMV), which belongs to the genus sobemoviruses. Sobemoviruses are single stranded positive sense RNA viruses which infect both mono and dicotyledonous plants. SeMV, originally isolated from the farmer’s fields in Tirupati, Andhra Pradesh, India infects Sesbania grandiflora plants that belong to the Fabaceae family. The SeMV genome is 4147 nucleotides (nt) long and encodes for three overlapping open reading frames (ORF). ORF 1 codes for the MP which is involved in cell to cell movement of the virus. The ORF3 encoded CP is translated via a subgenomic RNA synthesized during the viral replication. The CP assembles into icosahedral T=3 capsids composed of 180 chemically identical subunits which are arranged within 12 pentamers and 20 hexamers. In addition to encapsidation of the progeny genome, the CP was also shown to interact with MP and could therefore be involved in long distance movement. The central ORF 2 encodes for two polyproteins 2a and 2ab via ribosomal -1 frame-shifting mechanism. The polyprotein processing studies have established four cleavage sites within polyprotein 2a and three cleavage sites in polyprotein 2ab. The domain arrangement of polyprotein 2a is membrane anchor (MA)-Protease-VPg-p10-p8 while that of 2ab is MA-Protease VPg-RdRp. The N-terminal part of both these polyproteins codes for the viral protease which falls in the category of eukaryotic serine protease and has the ability to cleave the polyproteins both in cis as well as in trans. The C terminal domain of the polyprotein 2ab represents the viral RdRp which is the key catalytic component in the viral replication