Production of polyomavirus late mRNAs requires sequences near the 5' end of the leader but does not require leader-to-leader splicing.

Polyomavirus late mRNAs contain multiple copies of a 57-nucleotide leader sequence derived by splicing from multigenome-length late transcripts. Inefficient termination of transcription and inefficient polyadenylation allow accumulation of these giant transcripts. In this report, we show that a viable mutant virus, ins5, which contains an efficient rabbit beta-globin polyadenylation signal, produced late mRNAs whose vast majority contains only one leader. ins5 virus nevertheless produced as much late mRNA as did wild-type virus and grew as well as did wild-type virus in mouse cells. These results demonstrate that leader-to-leader splicing per se is not required for efficient production of late mRNAs or for efficient virus replication. However, we also found that RNAs lacking critical sequences near the 5' end of the leader did not accumulate as mRNAs and that most late transcripts made during the early part of the late phase, when few late mRNAs are produced, initiated downstream of the 5' end of the leader. These results indicate that a sequence element near the 5' end of the leader is required for proper processing, transport, or stability of late mRNAs and that the control of late mRNA production depends in part on the choice of transcription initiation sites at the late promoter.