Synthesis in vitro of the coat protein of papaya mosaic virus.

Papaya mosaic virus (PMV) RNA directs the synthesis in vitro in either the wheat germ or reticulocyte lysate translation systems of numerous products. Three polypeptides are most prominent: pA (155,000 molecular weight), pB (73,000), and pC (22,000). The last, pC, displays the same molecular weight as coat protein and is precipitated by antibodies raised against PMV. The identities of pA and pB are less clear, although peptide mapping suggests that they and many of the minor products of translation are related to each other, but not to pC. The ability of partially encapsidated PMV ribonucleoprotein particles to direct protein synthesis has been assessed. As the extent of encapsidation increased, the relative synthesis of pA and pB decreased markedly. In contrast, the synthesis of pC persisted until a substantial fraction of the PMV RNA was completely encapsidated. In view of the polarity of assembly of PMV in vitro (M. Abouhaidar and J. B. Bancroft, (1978), Virology 90, 54-59) this finding indicates that the coding sequences for PMV coat protein are localized toward the 3' end of PMV RNA. A kinetic analysis of coat protein synthesis, coupled with Northern blot analysis of PMV RNA during translation suggest that PMV RNA must be cleaved to activate it for in vitro coat protein synthesis.