Turnip yellow mosaic virus: biochemical aspects of the infection process

Abstract

Investigators have demonstrated that the RNA from a variety of plant viruses can function as mRNA in in vitro protein synthesising systems. An active homologous plant cell-free amino acid incorporating system has been isolated from Brassica chinensis L., leaves, and partially characterised employing polyuridylic acid(poly(U)) and turnip yellow mosaic virus(TYMV) RNA as messengers. Activity of this system has been compared with an active system isolated from developing seeds of Vicia faba L., and with mixed B. chinensis - V. faba systems. Bacterial contamination was found to be low in such incubations.[14(_c)] phenylalanine aminoacylation of B. chinensis tRNA was maximal at 17.5 mM Mg(^2+) under which conditions TYMV RNA was found to accept [14(_c)] valine. Maximum [14(_c)] phenylalanine poly(U) directed incorporation occurred at 8 mM Mg(^2+) in the V. faba and 6 mM Mg(^2+) in the B. chinensis system. TYMV RNA was relatively inactive when employing a single [14(_c)] amino acid, but was active in the B. chinensis system when employing a [14(_c)] amino acid mixture (maximum incorporation at 6mM Mg(^+2) and 70 mM K(^+)Partial characterisation of TYMV RNA (and other system components) was carried out using polyacrylamide gel and analytical ultracentrifugation techniques, indicating the infectious viral RNA to have a high sedimentation coefficient (c. 33 S).Various extraction procedures of plant and viral components are discussed with reference to ease of isolation and resultant biological activity. Microsomes isolated rapidly from young B. chinensis leaves were found to be most active in amino acid incorporation. The application of these experiments to studies on biochemical aspects of the TYMV infection process are considered in the light of other work in this field. It would appear that resistance of V. faba to infection by TYMV does not reside at the ribosomal level, and that TYMV protein synthesis occurs, at least in vitro, on the '80 S' rather than the '70 S' ribosome.