The great majority of viral mRNAs in mouse C127 cells transformed by bovine 
papillomavirus type 1 (BPV) have a common 3' end at the early polyadenylation 
site which is 23 nucleotides (nt) downstream of a canonical poly(A) consensus 
signal. Twenty percent of BPV mRNA from productively infected cells bypasses the 
early polyadenylation site and uses the late polyadenylation site approximately 
3,000 nt downstream. To inactivate the BPV early polyadenylation site, the early 
poly(A) consensus signal was mutated from AAUAAA to UGUAAA. Surprisingly, this 
mutation did not result in significant read-through expression of downstream 
RNA. Rather, RNA mapping and cDNA cloning experiments demonstrate that virtually 
all of the mutant RNA is cleaved and polyadenylated at heterogeneous sites 
approximately 100 nt upstream of the wild-type early polyadenylation site. In 
addition, cells transformed by wild-type BPV harbor a small population of mRNAs 
with 3' ends located in this upstream region. These experiments demonstrate that 
inactivation of the major poly(A) signal induces preferential use of otherwise 
very minor upstream poly(A) sites. Mutational analysis suggests that 
polyadenylation at the minor sites is controlled, at least in part, by UAUAUA, 
an unusual variant of the poly(A) consensus signal approximately 25 nt upstream 
of the minor polyadenylation sites. These experiments indicate that inactivation 
of the major early polyadenylation signal is not sufficient to induce expression 
of the BPV late genes in transformed mouse cells.