High level gene expression in mammalian cells by a nuclear T7-phase RNA polymerase.

Here we describe a novel expression system for mammalian cells which is based on transcription of hybrid genes containing T7 phage promoters by a T7 phage RNA polymerase targeted to the nucleus of the host cells. The RNA polymerase gene of T7 phage has been modified by substituting a sequence encoding the nuclear location signal of SV40 large T antigen for the N-terminal part of the polymerase gene. Expression of the modified gene is driven by the mouse metallothionein promoter in transfected mouse Ltk- cells resulting in high concentration of the polymerase in the nucleus. Nuclear T7 RNA polymerase directs efficient transcription of the cat gene under control of a T7 promoter. T7 constructs are expressed at a level at least 6 fold higher than the prototype pRSVcat. The unique properties of this heterologeous expression system are discussed.