The Homologous Regions of Spodoptera litura (Lepidoptera: Noctuidae) Nucleopolyhedrovirus II Have Both the Function as Origin of DNA Replication and Enhancer

In the genome sequence of the Spodoptera litura nucleopolyhedrovirus II (SpltNPVII), seven homologous regions (hrs), Sphr1-7, were identified. Each of them composed of three to eight 64-bp highly conserved sequences, and each contained a 24-bp imperfect palindrome. A transient expression assay demonstrated that the expression of SpltNPVII-ie1 promoter-driven luciferase gene was enhanced between 3- and 13-fold by infection of SpltNPVII in Spli221 cells. Real-time polymerase chain reaction confirmed each of seven hrs could function as origin (ori) of viral DNA replication. This suggests that these hrs are bifunctional, having both ori and enhancer activities for transcription. In addition, the potential of seven hrs as origins had a significantly positive correlation with the number of their palindromes (r = 0.847, Sig: 0.016 < 0.05), and enhancer efficiency had a significantly positive correlation with the number of characteristic motifs (r = 0.893, Sig: 0.007 < 0.01). The efficiency of replication and enhancement of each hr both increased with increasing total numbers of palindromes, repeat sequences, and characteristic motifs. In addition, a single 64-bp highly conserved consensus sequence cannot very good support to the function as origin and enhancer, and require the assistance of other cis-elements in hrs.