FIG. 5.

Tetracycline-regulated gene expression in the context of vaccinia virus infection. Cells were infected with the vtetR recombinant that expresses the TET repressor protein from a constitutive viral promoter. Cells were then transfected with one of three plasmids, pUC19, pInt/β-gal, and pLate/op/β-gal, and maintained in the absence (○) or presence (+) of 1 μg of TET per ml. At 24 hpi, cells were harvested and lysates were assayed for β-Gal activity using a colorimetric assay in which enzyme activity generates an increase in optical density at 420 nm (O.D. 420). One and 5 μl of each lysate were assayed. Values obtained from cells infected in the presence of TET are shown with black bars; those obtained from cells infected in the absence of TET are shown in hatched bars.