The Potential Role of a Late Gene Expression Factor, Lef2, fromBombyx moriNuclear Polyhedrosis Virus in Very Late Gene Transcription and DNA Replication

Abstract

Several late gene expression factors (Lefs) have been implicated in fostering high levels of transcription from the very late gene promoters ofpolyhedrinandp10from baculoviruses. We cloned and characterized fromBombyx morinuclear polyhedrosis virus a late gene expression factor (Bmlef2) that encodes a 209-amino-acid protein harboring a Cys-rich C-terminal domain. The temporal transcription profiles oflef2revealed a 1.2-kb transcript in both delayed early and late periods after virus infection. Transcription start site mapping identified the presence of an aphidicolin-sensitive late transcript arising from a TAAG motif located at −352 nucleotides and an aphidicolin-insensitive early transcript originating from a TTGT motif located 35 nucleotides downstream to a TATA box at −312 nucleotides, with respect to the +1 ATG oflef2. BmLef2trans-activated very late gene expression from bothpolyhedrinandp10promoters in transient expression assays. Internal deletion of the Cys-rich domain from the C-terminal region abolished the transcriptional activation. Inactivation of Lef2 synthesis by antisenselef2transcripts drastically reduced the very late gene transcription but showed little effect on the expression from immediate early promoter. Decrease in viral DNA synthesis and a reduction in virus titer were observed only when antisenselef2was expressed under the immediate early (ie-1) promoter. Furthermore, the antisense experiments suggested thatlef2plays a direct role in very late gene transcription.