A single nucleotide change in the coat protein gene of tobacco mosaic virus is involved in the induction of severe chlorosis.

YSI/1 is a mutant of the common strain (U1) of tobacco mosaic virus (TMV) which induces a severe yellow mosaic in Nicotiana tabacum instead of the light green/dark green mosaic induced by its parental U1 virus. Although there was less coat protein (CP) in whole leaf extracts of YSI/1-infected leaves than in U1-infected leaves, severalfold more CP was found in the chloroplasts, most of which was associated with the thylakoids. Sequencing the CP genes of both viruses showed the presence of nucleotide differences at viral RNA positions 5770 and 6127, both of which result in amino acid replacements; YSI/1 has an Asp-->Val change at amino acid 19 and a Ser-->Phe change at amino acid 138. A common strain TMV engineered to contain the YSI/1 3' end sequences, including the CP, induced the severe yellow mosaic of the YSI/1 mutant. A chimeric virus with the change only at nucleotide 5770 (amino acid 19) in the CP induced a severe yellow mosaic, showing that this replacement is involved in the induction of chlorosis by YSI/1. A second isolate of the same chimera also induced severe yellow mosaic symptoms; sequencing showed that it had gained the change at nucleotide 6127. However, a chimeric virus with the change only at nucleotide 6127 (amino acid 138) in the CP was unable to induce the severe yellow mosaic.