Characterization of chimeric turnip yellow mosaic virus genomes that are infectious in the absence of aminoacylation.

Previous experiments have characterized the chimeric genome TYMC-TMVPSK, in which the 3'-tRNA-like structure of turnip yellow mosaic virus (TYMV) was replaced by 3' sequences from tobacco mosaic virus. This genome accumulated in turnip protoplasts to a level about 3% of wild type, but was not infectious on plants. In the present study, TYMV sequences introduced into the anticodon loop and amino acid acceptor arm of the 3' region of this chimera led to three- to fourfold increases in viral accumulation. Two such modified chimeric genomes gave rise to stable infections in plants. After further passaging in plants and the accumulation of minor sequence changes in the 3' terminal region, the resultant viruses, TYMC-XX and TYMC-YY, were highly infectious. Viral accumulations in protoplasts were about 40% of wild type on the basis of coat protein levels, and virion yields in plants were about 0.1 mg/g leaf. Extensive assays failed to detect aminoacylation of these genomic RNAs in vitro, but they were active substrates for wheat germ. CCA nucleotidyltransferase. In separate experiments, the 3'-tRNA-like structure of TYMV RNA was replaced by the 3' terminal 96 nucleotides from erysimum latent tymovirus RNA, resulting in a genome that was infectious to plants (isolate TYMC-H). This chimeric virus produced similar symptoms and virion yield in plants as TYMC-XX and -YY, although accumulations of coat protein in protoplasts were 13% of wild type. The viral RNA was a poor substrate for CCA nucleotidyltransferase and could not be aminoacylated. TYMC-XX, -YY, and -H are the first TYMV replicons known to amplify efficiently and infect plants in the absence of aminoacylation. Their viability suggests that other properties can compensate for the absence of aminoacylation.