FIG. 1.

Concept of inducible trans complementation. In Escherichia coli the BAC pSM3fr-ΔM94 was generated by insertion of the tTA transactivator cassette into pSM3fr, thereby deleting M94. The trans-complementing cell line NT/M94-7 expresses pM94 under the control of the Tet-inducible promoter. Upon transfection with pSM3fr-ΔM94, expression of tTA by the viral genome induces expression of pM94 by the cell, leading to the production of trans-complemented MCMV-ΔM94. This virus is able to infect noncomplementing first-target cells. Due to the lack of the essential gene M94, the release of infectious virus particles is impossible although immediate early (IE), early (E), and late (L) viral gene expression as well as DNA replication (DNA rep) occurs.