Promoter Activities in the Baculovirus Envelope Glycoprotein gp64 Gene

( ¹Key
Laboratory of Silkworm Biotechnology, Ministry of Agriculture, Sericultural
Research Institute,
Chinese Academy of Agricultural Sciences, Zhenjiang 212018, China; 2 State
Key Laboratory of Bioreactor Engineering, East China University of Science
and Technology, Shanghai 200237, China; ³Institute of Biochemistry
and Cell Biology, Shanghai Institutes for Biological Sciences, the Chinese
Academy of Sciences, Shanghai 200031， China )

Abstract
Baculovirus GP64 envelope glycoprotein is a specific major component
of the envelope of the budded virus and is involved in virus entry into the
host cells by endocytosis. For promoter activity analysis in the baculovirus
gp64 gene, two DNA fragments containing 437 and 439 bp upstream of
5′ ends of the BmNPV and AcMNPV gp64 ORF were amplified by polymerase
chain reaction and cloned, respectively. The sequence analysis indicated that
two gp64 genes have both early (CAGT) and late (A/GTAAG) transcriptional
start sites. By use of the plasmids with a reporter luciferase gene (Luc)
driven by gp64 promoter to transfect insect cells, transient expression
assay showed that pBmgp64Luc had high expression levels in permissive
Bm-N cells and very low levels in non-permissive Sf-21 cells, while pAcgp64Luc
had relatively high expression levels both in permissive Sf-21 cells and in
non-permissive Bm-N cells. Furthermore, the transcription of both gp64
promoters appeared to be transactivated by 2.4-4 times in corresponding permissive
cells by corresponding viral factors, separately. By inserting BmNPV homologous
region-3 (hr3) into the downstream of luciferase reporter gene driven
by gp64 promoter, it enhanced transcription from both gp64 promoters
by 13-22 times in Bm-N cells and over 7000-14 000 times in Sf-21 cells, respectively.
In the presence of BmNPV hr3, correspondingly, the viral factors transactivated
the transcriptional activity from two promoters by about 73-78 times in corresponding
permissive cells. It suggested that BmNPV hr3 plays an important role
in co-activation with viral factors onto the gp64 promoter besides
the functions of viral DNA origin and enhancer.