The papillomavirus E2 protein is a critical viral regulatory protein with 
transcription, DNA replication, and genome maintenance functions. We have 
previously identified the cellular bromodomain protein Brd4 as a major 
E2-interacting protein and established that it participates in tethering bovine 
papillomavirus type 1 E2 and viral genomes to host cell mitotic chromosomes. We 
have also shown that Brd4 mediates E2-dependent transcriptional activation, 
which is strongly inhibited by the disruption of E2/Brd4 binding as well as by 
short hairpin RNA (shRNA) knockdown of Brd4 expression levels. Since several 
mutants harboring single amino acid substitutions within the E2 transactivation 
domain that are defective for both transcriptional transactivation and Brd4 
binding are also defective for transcriptional repression, we examined the role 
of Brd4 in E2 repression of the human papillomavirus E6/E7 promoter. 
Surprisingly, in a variety of in vivo assays, including transcription reporter 
assays, HeLa cell proliferation and colony reduction assays, and Northern blot 
analyses, neither blocking of the binding of E2 to Brd4 nor shRNA knockdown of 
Brd4 affected the E2 repression function. Our study provides evidence for a 
Brd4-independent mechanism of E2-mediated repression and suggests that different 
cellular factors must be involved in E2-mediated transcriptional activation and 
repression functions.