Enhanced splicing of nonconsensus 3' splice sites late during adenovirus infection

The adenovirus major late transcription unit is an example of an alternatively spliced gene, in which a common 5' splice site can be spliced to more than 15 different 3' splice sites. Here we show that the specificity in 3' splice site recognition changes during virus infection, such that 3' splice sites with long consensus-type polypyrimidine tracts are repressed while 3' splice sites with short atypical polypyrimidine tracts, which bind U2AF65K inefficiently, are enhanced in splicing in late virus-infected nuclear extracts. On the basis of these experiments, we discuss a mechanism that helps to explain how the complex pattern of major late mRNAs is produced late during virus infection.