Differentiation-dependent chromatin rearrangement coincides with activation of human papillomavirus type 31 late gene expression

The life cycle of human papillomaviruses (HPVs) is tightly linked to the di fferentiation status of the host cell. While early genes are expressed duri ng the initial stages of viral infection, late gene expression occurs in th e suprabasal layers of the cervical epithelium. Late genes encode E1<^>E4, a cytosolic protein, and capsid proteins L1 and L2. We have mapped over 30 initiation sites for late transcripts and show that the transcripts initiat e in a 200-nucleotide region within the E7 open reading frame. The mechanis ms regulating the activation of late gene expression, however, are not yet understood. DNase I hypersensitivity analysis of HPV-31 chromatin in cell l ines that maintain viral genomes extrachromosomally indicates that a major shift in nuclease digestion occurs upon differentiation. In undifferentiate d cells, hypersensitive regions exist in the upstream regulatory region pro ximal to the E6 open reading frame. Upon differentiation, a region between nucleotides 659 and 811 in the E7 open reading frame becomes accessible to DNase I. These results indicate that the late transcript initiation region becomes accessible to transcription factor binding upon differentiation. Se veral complexes mediate chromatin rearrangement, and we tested whether hist one acetylation was sufficient for late transcript activation. Treatment wi th the histone deacetylase inhibitor trichostatin A was found to be insuffi cient to activate late gene expression in undifferentiated cells. However, it did activate expression of early transcripts. These results suggest that chromatin remodeling around the late promoter occurs upon epithelial diffe rentiation and that mechanisms in addition to histone deacetylation contrib ute to activation of late gene expression.