Synthesis of Black Beetle Virus Proteins in Cultured Drosophila Cells: Differential Expression of RNAs 1 and 2

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Abstract
Black beetle virus is an insect virus with a split genome consisting of 2 single-stranded, messenger-active RNA molecules with MW of 1.0 .times. 106 (RNA 1) and 0.5 .times. 106 (RNA 2), respectively. Virions contained 2 proteins, .beta. with a MW of 43,000 (43 K) and .gamma. (5 K), and traces of a 3rd protein, .alpha. (47 K). When translated in cell-free extracts of rabbit reticulocytes, RNA 1 directed the synthesis of protein A (104 K); RNA 2 synthesized protein .alpha.. The in vitro translation efficiency of the 2 RNA was roughly equal. Infection of cultured Drosophila cells induced the synthesis of 5 new proteins: A, .alpha., .beta., .gamma. and B (10 K), detected by autoradiography of polyacrylamide gels after electrophoresis of extracts from [35S]methionine-labeled cultures. All but protein .gamma. could also be detected by staining with Coomassie brilliant blue, indicating vigorous synthesis of viral proteins. Pulse-chase experiments in infected cells revealed the disappearance of protein .alpha. and the coordinate appearance of proteins .beta. and .gamma., supporting an earlier proposal that coat protein of mature virions is made by cleavage of precursor .alpha.. Proteins A and B were stable in such pulse-chase experiments. The 3 classes of virus-induced proteins, represented by A, B and .alpha., were synthesized in markedly different amounts and with different kinetics. Synthesis of proteins A and B peaked early in infection and then declined; synthesis of coat protein precursor .alpha. peaked much later. RNA 1 apparently controls early replication functions via protein A (and also possibly protein B); RNA 2 controls synthesis of coat protein required later for virion assembly.