A composite polyadenylation signal with TATA box function

A variant polyadenylation signal, which is conserved and employed by mammal ian hepadnaviruses, has a sequence resembling that of the TATA hox. We repo rt here that this composite box manifests all the promoter characteristics. It binds effectively TATA-binding protein with TFIIB and TFIIA in a synerg istic manner. This capacity, however, is lost when the box is converted to a canonical and simple poly(A) signal. Furthermore, we show that it has pro moter activity and supports transcription of reporter genes preferentially in liver-derived cells, a characteristic behavior of the hepatitis B virus (HBV) promoters. In addition, we show that the HBV noncanonical poly(A) sig nal supports transcription initiation from the viral genome, suggesting tha t it is a genuine promoter, possibly of the polymerase/reverse transcriptas e gene, Finally, we found that this deviant poly(A) signal is crucial for H BV replication since a viral mutant with a canonical poly(A) box is impaire d in replication. Our data, therefore, raise the interesting and novel poss ibility that a composite poly(A) box might have a dual function. At the lev el of DNA ft functions as a promoter to initiate transcription, whereas at the level of RNA it serves as a poly(A) signal to process RNA. An interesti ng outcome of this strategy of gene expression is that it provides a novel mechanism for the synthesis of an approximately genome length transcript.