Transcriptional activity of inverted terminal repeats of various human adenovirus serotypes

Fifty-three human adenovirus serotypes were identified, which are divided into seven subgroups A, B1, B2, C, D, E and F. All types of recombinant adenoviruses have serotype-specific left and right inverted terminal repeats (ITRs). There is evidence that sequences in the ITRs of subgroup C exhibit promoter activity, which in turn might influence the expression of coding sequences that are in close proximity. We investigated whether ITRs from the complete spectrum of adenovirus subgroups show transcriptional activity. We found that ITRs from subgroups A, C and F cloned in a forward orientation display robust transcriptional activity in a cell-type independent manner. In the reverse orientation only subgroup B2 showed transcriptional activity. Unexpectedly, we also found that most ITRs when located upstream of a ubiquitously active promoter drastically reduced reporter gene expression, suggesting that ITRs may have a repressive activity on transcription.