Promoter interactions in retrovirus vectors introduced into fibroblasts and embryonic stem cells

The activity of the Moloney murine leukemia virus promoter is restricted in mouse embryonic stem cells. Gene expression with retrovirus vectors can be achieved in these cells if internal promoters are used. To address the possible influence of the viral enhancer sequences on expression from the internal promoter, we have constructed high-titer, self-inactivating retrovirus vectors which delete viral regulatory sequences upon integration in the host genome. We show that deleting most of the viral enhancer sequences has no significant effect on viral titer. This enhancer deletion leads to either an increase or a decrease in the amount of RNA transcribed from the internal promoter, but no consistent change can be found with any type of vector. The same changes in expression from the internal promoter observed in embryonic stem cells are also observed in 3T3 fibroblast cells, in which the viral promoter is active. These results indicate that viral regulatory elements influence expression from an internal promoter independently of expression from the virus promoter.