III. Effects of Tobacco Mosaic Virus Mutants on Protein Synthesis in Nicotiana fubacum

Nicotiana tabacum, infected with three widely differing chemically produced and one natural mutant of tobacco mosaic virus (TMV), was pulse labeled with [“HJleutine. Uninfected plants were similarly pulse labeled with [i4C]leucine. The 3H- and W-labeled leaf tissues were combined, fractionated into subcellular fractions, denatured, dialyzed, and electrophoresed on polyacrylamide gels in SDS. Increases or decreases in the 3H/r4C ratio in gel slices indicated changes in the proteins synthesized in the infected fractions as compared to the uninfected control fractions. In contrast to wild-type TMV, where only coat protein was detected (Singer, 1971), protein synthesis differed markedly among the mutants and their subcellular fractions in both the number and molecular weights of the protein peaks. No protein was found to be common to all strains in the same subcellular fractions except the coat protein. In addition, decreases in the 3H/i4C ratio were found, indicative of suppression of specific host proteins. The total molecular weights of proteins appearing upon infection with some mutants exceeded the coding capacity of the viral nucleic acid, and there was no indication that any of the very large proteins found were precursors. It appears probable that these multiple responses specifically produced by mutants are the result of changes in host protein synthesis caused by the infection.