Characterization of an early gene accelerating expression of late genes of the baculovirus Autographa californica nuclear polyhedrosis virus

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Abstract
The region of the Autogrpha californica nuclear polyhedrosis virus (AcMNPV) encompassing the EcoRIT fragment (29.0 to 30.1 map units) was characterized by DNA sequencing, transcriptional mapping, and site-directed mutagenesis. The largest transcript from this region, an early 1.7-kilobase (kb) poly(A)+ RNA, encompassed three tandem, nonoverlapping open reading frames (ORFs). The largest of these ORFs, ETL, was proximal to the 5'' end of the transcript and had the capacity to encode a 28-kilodalton (kDa) polypeptide. A recombinant virus, vETL.beta.gal, containing the Escherichia coli coli .beta.-galactosidase (.beta.gal) gene fused to the N-terminal two-thirds of the ETL ORF, produced blue plaques in the presence of a chromogenic indicator of .beta.gal and wild-type levels of polyhedra in cell culture. This recombinant was also infectious in insect larvae by oral administration of occluded virus. Comparison of vETL.beta.gal and wild-type viral proteins pulse-labeled at various times postinfection (p.i.) revealed (i) absence of a virus-induced 28-kDa polypeptide, (ii) early expression of a large (approximately 130-kDa) polypeptide which may be the ETL-.beta.gal fusion protein, (iii) a delay in expression of early 35 and 40-kDa polypeptides, and (iv) a 4- to 6-h delay in the expression of late proteins in vETL.beta.gal-infected cells. Cycloheximide did not inhibit synthesis of the 1.7-kb RNA but did inhibit its shutoff, which occurs at 12 h p.i. in the absence of inhibitors. Thus, the ETL gene product is apparently an early 28-kDa protein which is necessary, directly or indirectly, for timely expression of many other AcMNPV genes. The promoter-leader region of the 1.7-kDa transcript showed significant sequence similarities to the leader of the AcMNPV IE-1 gene. The middle ORF within the 1.7-kb transcript, ETM, would encode a hydrophobic polypeptide of 113 amino acid residues. ETS, a small ORF within and proximal to the 3'' end of the 1.7-kb transcript, was also transcribed as a set of smaller (approximately 0.5-kb) RNAs initiated heterogeneously in the region between ETL and ETS and persisting throughout infection.