THE SATELLITE TOBACCO NECROSIS VIRUS: A SINGLE PROTEIN AND ITS GENETIC CODE

Abstract

The satellite tobacco necrosis virus (STNV) cannot multiply in its plant hosts in the absence of tobacco necrosis virus (TNV) to which it is serologically and morphologically unrelated. An explanation of this deficiency was sought through studies of its structure and composition. STNV was found to be a nucleoprotein having a molecularweight of 2X106, containing 20 per cent RNA. The RNA consisted of only 1200 nucleotide residues. The nucleotide composition was within the normal range of naturally occuring RNA and no abnormal bases were detected. The molecular weight of the protein subunit was 39,000, derived from determinations of minimum molecular weight in conjunction with the number of peptides obtained after tryptic digestion and fingerprinting. End-group determination with fluorodinitrobenzene, leucine aminopeptidase, and carboxypeptidase confirmed this value. Not counting tryptophan, which has not been determined, the protein subunit was found to be made up of 372 amino acid residues. Serological tests on sap from plants infected with TNV only demonstrated that TNV did not produce antigens related to the STNV protein. This suggested that TNV did not code for the STNV coat protein, and it was concluded that STNV itself codes for its own protein. According to current ideas on genetic coding, the STNV-RNA could code for a maximum of 400 amino acid residues, which is consistent with the number of amino acid residues in the coat protein as estimated from the present data.