Promoter activity of left inverted terminal repeat and downstream sequences of porcine adenovirus type 3

Early region 1 (E1) of porcine adenovirus type 3 (PAdV-3) consists of E1A and E1B transcription units. The authentic promoter region of E1A contains a TATA box at nucleotide position (nt) 449 and a bifunctional regulatory element between nt 374 and 431, which enhances the transcription of E1A, but represses that of E1B. Here, we investigated the role of the left inverted terminal repeat (ITR) and its downstream sequences (between nt 151 and 312) in the transcription of early viral genes, and viral replication. Mutant PAdV-3s without the authentic E1A promoter region could be rescued by transfection of mutant genomic DNA into fetal porcine retina cells. Moreover, the mutant PAdV-3s produced E1A-specific mRNA and remained viable in swine testis (ST) cells suggesting that the left-terminal 151 bp including the ITR, can serve as a promoter for E1A expression. However, mutant PAdV-3s containing deletion including authentic E1A promoter region, displayed both reduced steady-state levels of early gene mRNAs (E1A, E1B, E2A, E3, and E4) and decreased rate of viral replication in ST cells. Interestingly, mutant PAdV-3s containing the left-terminal 312 bp displayed increased transcription of early genes including E1A. Our results suggest that the left ITR of PAdV-3 contain the promoter like elements and the sequences (between nt 151 and 312) downstream of left ITR can enhance its promoter activity.