Characterization of late gene transcripts expressed during vegetative replication of human papillomavirus type 31b

Human papillomaviruses (HPVs) are etiologic agents of anogenital cancers. The lack of an efficient in vitro system with which to study the differentiation-dependent vital life cycle has impeded most investigations of viral transcription and gene expression. The CIN-612 clone 9E cell line latently maintains episomal copies of HPV type 31b (HPV31b). The complete replicative life cycle of HPV31b can be studied by using the organotypic (raft) culture system. A number of spliced HPV31b early gene transcripts and two late gene transcripts have been described in studies using the raft system. An HPV31b early promoter, P₉₇, and a differentiation-induced promoter, P₇₄₂, have been characterized by using this system. In this study, we used the raft system to analyze the temporal expression patterns of HPV31b late gene transcripts during the viral life cycle. The expression of late RNAs peaked at day 12 after lifting to the air-liquid interface; the levels then declined dramatically by day 16. The peak of late RNA expression was coincident with the appearance of virus particles in the raft tissues. We characterized transcripts with the potential to encode late gene products, including 19 RNAs containing the L1 region and 4 RNAs containing the E5b and L2 open reading frames. We also found evidence for two novel promoters. Transcription of both L1- and L2-containing RNAs initiated at a region upstream of the early promoter. In addition, late gene RNAs were also transcribed by using a promoter in the E4 reading frame.