Detection of an element of the SV40 late promoter in vectors used for expression studies in COS cells.

Plasmids containing hepatitis B virus (HBV) DNA and a 232-bp SV40 DNA fragment encoding the origin of replication were constructed. When introduced by transfection into COS cells, these plasmids directed the synthesis of hepatitis B surface antigen. S1 mapping of the mRNAs covering the S gene showed that transcriptional initiation was promoted by the interaction of HBV sequences with an SV40 promoter element: transcription started on HBV DNA but had several properties of SV40 late transcription. The detection of a promoter element in an SV40 origin fragment commonly used in the COS system is important for the interpretation of data deriving from expression studies in COS cells.