NUCLEOTIDE-SEQUENCE AND IN-VITRO EXPRESSION OF THE CAPSID PROTEIN GENE OF TOBACCO RINGSPOT VIRUS

The nucleotide sequence of the 3' terminal 2022 nucleotides (nt) of tobacco ringspot virus (TobRV) RNA 2 has been determined. Protein microsequence analysis of the amino-terminal residues of purified capsid protein localized the capsid protein gene between nt 2014 and 583 (from the 3' terminus) of this sequence. The proteolytic cleavage site that is processed to liberate the capsid protein from the RNA 2-encoded polyprotein was identified as Cys-Ala. The predicted translation product from the gene is a 477 amino acid long polypeptide with a calculated MW of 53 kDa. The gene was modified at the 5' end to facilitate sub-cloning, and to provide it with a methionine initiation codon. The modified gene was sub-cloned, transcribed in vitro and expressed in a rabbit reticulocyte lysate translation system, where it directed the synthesis of a 53 kDa polypeptide. Gamier-Osguthorpe-Robson analyses of the secondary structure of the capsid protein predicted the presence of three beta sheet domains, which suggests that this nepovirus capsid may be structurally analogous to those of the como- and picornaviruses. These and other results from computer analyses of the nucleic acid and amino acid sequences, and comparisons with the capsid proteins of nepoviruses and other related viruses are discussed.