Promoter motifs essential to the differential transcription of structural and non-structural genes of the white spot syndrome virus.

Chimeric reporter genes were generated comprising nine different promoters of the white spot syndrome virus linked to luciferase, with the aim to compare their transcriptional activities in insect cells. The promoters included the four non-structural genes DNA polymerase, ribonucleotide reductase small subunit, ribonucleotide reductase large subunit, and thymidine-thymidylate kinase, and the five structural genes VP15, VP19, VP24, VP26, and VP28. The promoters of the non-structural but not the structural genes can function in these cells, indicating that transcription of the non-structural genes can be recognized by cellular transcriptional machineries. While the structural genes were highly expressed in natural host cells during white spot syndrome virus infection, their promoters failed to direct transcription in insect cells, suggesting that transcription of these late genes may require virally induced host factor(s). Motifs essential for transcription of the above non-structural genes were identified by transient transfection of insect cells with constructs containing a series of deletions in the 50 terminal region and within the promoter. The minimal promoter sequences of these four genes were also capable of driving expression of the enhanced green fluorescent protein in insect cells.