Virus-ribosome complexes from cell-free translation systems supplemented with cowpea chlorotic mottle virus particles

When particles of cowpea chlorotic mottle virus (CCMV) were added to cell-free extracts from wheat germ, the encapsidated viral genome was translated into polypeptides similar to the translation products specified by unencapsidated viral RNA (as shown before by M.J. Brisco, R. Hull, and T.M.A. Wilson, 1986, Virology 148, 210-217). The rate of protein synthesis observed upon addition of virus particles was much slower than that of extracted RNA and the quantity of protein formed was only 10% of that of extracted RNA. Using sucrose and cesium-chloride gradient analysis, virus-ribosome complexes, containing up to four ribosomes per virus particle, were isolated from translation mixtures supplemented with CCMV particles. These complexes, with densities intermediate of those of virus (1.36 g cm-3) and ribosomes (1.58 g cm-3), were analyzed and quantified in the electron microscope. Less than 5% of the particles was found in association with ribosomes. To verify whether these complexes were involved in the process of cotranslational disassembly, tobacco mosaic virus was analyzed with the same techniques and methods. The results found for TMV were similar to those found for CCMV except that virus-ribosome complexes with up to 20 ribosomes per virus particle were observed. The implications of the process of virion-directed translation for the structure of the particle as well as the role of this process in vivo are discussed.