The life cycle of human papillomaviruses (HPVs) is tied to keratinocyte differentiation. One key event in the viral life cycle is the differentiation-dependent increase in vital replication. This increase in replication activity results in an amplification of the HPV genome from approximately 50 copies per cell in basal keratinocytes to thousands of copies of the vital genome per cell in suprabasal keratinocytes. To characterize the events associated with this differentiation-dependent increase in HPV replication, we have initiated studies of mRNAs encoding the HPV replication protein E1 during the differentiation of cell lines that stably maintain episomal HPV DNA. Differentiation induced the expression of several transcripts that hybridized to an E1-specific probe. One of these messages, a 3.7-kb transcript, did not hybridize to a probe specific for the early promoter upstream of the E6 open reading frame. RNase protection analysis confirmed an induction of unspliced messages derived from the differentiation-dependent promoter at nucleotide 742 in the middle of the E7 open reading frame. These observations demonstrate a differentiation-induced increase in E1 mRNAs derived from the vital late promoter and suggest a role for increased E1 expression during amplification of the HPV genome.
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