Identification of the virus-induced protein of tobacco leaves that resembles the sweet-protein thaumatin

One of the pathogenesis-related proteins, PR-R, induced in tobacco leaves by infection with tobacco mosaic virus, has many of the properties expected of the virus-induced, thaumatin-like protein that has been predicted from mRNA studies. It has a high content of cysteine, a molecular weight near 23 000, and one of its two isoforms has a sequence of 54 amino acids that is identical to that in the predicted protein. Although the circular dichroism spectrum of PR-R resembles that of the sweet-tasting protein thaumatin, the protein does not appear to react with a thaumatin antibody, nor has it a marked sweet taste.