Functional characterization of putative promoter elements from infectious hypodermal and hematopoietic necrosis virus (IHHNV) in shrimp and in insect and fish cell lines

Infectious hypodermal and hematopoietic necrosis virus (IHHNV) of shrimp contains a linear single-stranded DNA genome of approximately 4.1 kb with three putative open reading frames (ORFs) on the same DNA strand designated, the Left, Middle, and Right ORFs. The Left ORF codes for non-structural protein and the Right ORF codes for capsid protein, whereas the role of the Middle ORF is still unknown. Two putative promoters, designated P2 and P61, were detected upstream of the Left ORF and Right ORF, respectively. We evaluated the activities of these two promoters with or without a transcriptional enhancer element via the use of firefly luciferase reporter constructs in insect and fish cells, and in shrimp tail muscle. In insect and fish cells, the P2 promoter was stronger than the P61 promoter. The presence of the SV40 enhancer element negatively affected P2 but not P61 promoter activity in insect cells. However, in fish cells, the SV40 enhancer element dramatically increased the activities of both promoters. In shrimp, there was no significant difference in luciferase expression driven by these two promoters. In shrimp tail muscle, the presence of SV40 enhancer element in the construct had no significant effect on the P2 promoter and a negative effect on the P61 promoter. The IHHNV P2 and P61 promoters were found to be constitutive promoters that can drive gene expression in both invertebrate and vertebrate hosts.