Tobacco Mosaic Virus Ribonucleic Acid-dependent Amino Acid Incorporation in a Wheat Embryo System in Vitro

Abstract Incubation of radioactive tobacco mosaic virus RNA (TMV-RNA) with a wheat embryo ribosome-supernatant system depleted of transfer RNA results in the formation of a new radioactive component which sediments in a sucrose density gradient somewhat faster than 80 S ribosomes. The reaction requires the presence of ATP, two supernatant factors, and ribosomes. The gradient position of the new radioactive component and the participation of ribosomes in its formation indicate that it is a ribosome-TMV-RNA complex. The conditions and requirements for formation of the complex are identical with those necessary for removing the kinetic lag in aminoacyl-tRNA incorporation. The complex may therefore be described as a ribosome-messenger initiation complex. Formation of the ribosome-TMV-RNA complex is unaffected by addition of cycloheximide or puromycin, nor is radioactive tRNA incorporated into it. Thus, neither free nor aminoacylated-tRNA appears to be a component of the complex. When tRNA is added to the incubation, thereby making aminoacyl transfer possible, radioactivity from the primary complex is chased into polyribosome components sedimenting more rapidly in the gradient. This latter reaction is inhibited by puromycin or cycloheximide. These observations suggest that the TMV-RNA-ribosome complex is a direct precursor of the polyribosome and that the conversion of this complex into polyribosomes requires aminoacyl transfer.