Multiple AT‐rich sequences function as a cis‐element in the ORF3 promoter in channel catfish virus (Ictaluridherpesvirus 1)

The expression of herpesvirus genes during infection of tissue culture cells can be classified into three main classes: immediate‐early (IE), early and late. The transcriptional regulation of herpesvirus IE genes is a critical regulatory step in the initiation of viral infection, with their regulation differing from that of early and late genes. Herein, we report that an IE gene (ORF3) promoter in channel catfish virus (CCV, Ictalurid herpesvirus 1) can be activated regardless of the presence or absence of CCV infection, indicating that the ORF3 promoter is efficiently driven by host–cell transcription factors in a viral infection‐independent manner. The analysis of truncated promoter activity suggested that several transcription elements play a role in activating the ORF3 promoter, with the key cis‐elements seemingly located in the flanking sequence of the start codon ATG. We further found that this flanking sequence contained multiple AT‐rich sequences, and systematic mutational analyses showed that these AT‐rich sequences affected normal transcription levels of the ORF3 promoter. To summarize, multiple AT‐rich domains, representing the novel architecture of IE gene promoters in Ictalurid herpesvirus 1, serve as a cis‐element for ORF3 transcription.