The occurrence of the coat protein messenger RNA in the minor components of turnip yellow mosaic virus

Abstract

Previous studies from this laboratory [l] showed that the infectivity of turnip yellow mosaic virus (TYMV) is confined to RNA molecules with a M, of 1.9 X 106. These molecules resist heat treatment at 65°C [l] and solving in 98% formamide (unpublished results). This strongly suggests that the genome of TYMV consists of an uninterrupted RNA chain with a M, of 1.9 X 1 06. In vitro translation of this RNA in a cell-free extract of wheat germ yields none or very little coat protein, which means that the coat protein cistron of the genome RNA is closed under our in vitro conditions. Very efficient synthesis of coat protein occurs, however, with a small RNA (M, 240 000) which can be isolated from a virus preparation. A similar coat protein messenger was observed and described by Klein et al. [2]. Since this active messenger comprises about 1% of the unfractionated RNA, not all of the virus particles will contain the coat protein messenger. TYMV preparations contain in addition to the main infectious component B, a number of minor nucleoprotein components (Bo, Boo and Booo), the origin and function of which are as yet unknown [3-71. When fractionated in a CsCl density gradient B1 can be partly converted to a Bz particle with a higher buoyant density. A more thorough analysis by Mathews [8] revealed the presence of double bands for each of the five nucleoprotein components described above. Considering this heterogeneity of TYMV the question arises in which of these components the coat protein messenger is present. Isolation of the various components of TYMV with CsCl density gradient centrifugation and in vitro translation of the extracted RNAs in a cell-free extract of wheat germ revealed that none or very little coat protein is synthesized with the RNAs from the main components Bla and Bza (compare fig.1). Coat protein synthesis is found, however with RNA from particles with densities between that of Bla and Bza and higher than that of Bza. The minor components Boo and Booo also contain the messenger RNA for the coat protein.