Molecular cloning and characterization of a late Tipula iridescent virus gene

Virions of the cytoplasmic, icosahedral insect virus, Tipula iridescent virus (TIV), contain two major DNA components (L, greater than 176 kb; and S1, 10.8 kb) and 25-30 proteins. We characterized a gene (L96) whose 3.6-kb transcript is expressed late in the course of TIV infection of cultured of Estigmene acrea (salt marsh caterpillar, permissive host) and Aedes albopictus (mosquito, semipermissive host) cells. The L96 gene has an open reading frame of 867 codons, predicting a protein of 96 kDa with a pI of 10.9. The C terminus of the L96 protein is rich in hydrophobic amino acids and contains a small region of homology spanning a proteolytic cleavage site within two mammalian viral (GAG) polyproteins. Additional identity with H5 lysine-rich histones in the same region and with other DNA-binding proteins suggests that this protein may be involved in TIV structure. The lengths of the 5'- and 3'-untranslated regions of the L96 transcript were determined to be 21 nucleotides (nt) and 700 nt, respectively. Comparison of the TIV L96- and capsid-encoding genes, both of which are expressed late in infection, revealed that their 5' and 3' regions are generally rich in A and T residues, and that their 3' ends encode at least one eukaryotic polyadenylation signal (AATAAA).