FIG. 1.

Diagrams of the Tet-ON and the Prog systems. A mixed-infection strategy is used to introduce the activator and the CAT reporter genes to recipient cells. Reporter gene expression is then activated by the addition of the respective inducer. The Tet and Prog enhancer sequences were cloned upstream of a minimal major late TATA promoter element. We fused a cDNA encoding the major late tripartite leader immediately downstream of the transcription start site, as a 5′ noncoding sequence. The tripartite leader has been shown to function as an mRNA export signal (8) and a translational enhancer (15) in late-infected cells. The gene cassettes were inserted into the E1 region of Ad5 dl309 (9), making the recombinant adenoviruses replication deficient.