A novel capsid expression strategy for Thosea asigna virus (Tetraviridae)

This paper presents evidence that Thosea asigna virus (TaV) has a unique ca psid expression strategy and is a member of the Nudaurelia beta-like genus of the Tetraviridae. Electron microscopy of TaV particles indicated a 38 nm , T = 4 icosahedral capsid similar in structure to that of Nudaurelia beta virus (N beta V). TaV particles have a buoyant density of 1.296 g/cm(3) in CsCl and consist of two capsid proteins of 56 and 6 kDa. The virus genome c ontains a genomic RNA molecule of 6.5 kb and a subgenomic molecule of 2.5 k b. Northern blotting of TaV RNA indicated a genomic organization similar to that of NPV. The capsid gene of TaV is carried on both the genomic and sub genomic RNA molecules, while the RNA polymerase gene is present only on the genomic RNA. Cloning and sequencing of the TaV capsid gene identified an o pen reading frame that could potentially encode a capsid precursor protein of up to 82.5 kDa. The N-terminal sequences of the capsid proteins were com pared with the nucleotide sequence of the capsid open reading frame. The se quences indicate that the pre-protein is cleaved at two positions to produc e the 56 and 6 kDa capsid proteins as well as a predicted third protein tha t was not detected in the mature virion. Phylogenetic analysis of the capsi d proteins indicated that TaV is more closely related to NPV than to the Nu daurelia omega-like viruses. The eight beta-sheets that make up a jelly rol l structure in the TaV capsid protein were identified by computer analysis.