The papillomaviruses cause benign and malignant lesions of squamous epithelia in higher vertebrates. The complete lytic cycle of these viruses (including late gene expression) occurs only in the differentiated cells of the squamous epithelium. Malignant lesions and infected cells in culture do not produce virus. Bovine papillomavirus type 1 (BPV-1) was used as a model system for the study of papillomavirus late gene expression and its control. The data suggest that late gene expression is regulated at multiple transcriptional and post-transcriptional levels. BPV-1 mutants as well as eukaryotic expression vectors have been used to show that the BPV-1 late polyadenylation site can be efficiently utilized, even in transformed cells, and that the late poly(A) site is a more efficient poly(A) site than the early poly(A) site. The inefficient use of the late poly(A) site in its normal context may be due partially to transcriptional pausing between the early and late poly(A) sites in transformed cells and partially to a short inhibitory element in the late 3' UTR. This late 3' UTR element was shown not to function by destabilizing late mRNAs and may function at an RNA processing or transport level. There is also evidence that the BPV-1 poly(A) site choice may be regulated by splicing factors. Current data suggest that utilization of an alternative splice acceptor at nt 3225 coupled with enhanced recognition of a weak nonconsensus splice donor at nt 3764 are critical events in the early to late shift. It was shown, using a late minigene expression vector driven by a heterologous promoter, that mutation of the nt 3225 splice acceptor, forcing use of the nt 3605 splice acceptor, mutation of the nt 3764 splice donor to a strong consensus sequence, and vegetative replication are both necessary and sufficient to give a late pattern of RNA processing. Furthermore, the link between splice acceptor choice and utilization of the nonconsensus splice donor is due to exon size, consistent with the exon recognition model of splicing.
