Conditional expression of foreign genes by temperature-sensitive mutants of vaccinia virus

To assess the utility of two temperature-sensitive (ts) mutant vaccinia viruses as vectors for the conditional in vitro expression of recombinant foreign genes, we have studied the kinetics of expression of foreign genes incorporated into these viruses. At nonpermissive temperature, 40 degrees C, these viruses were defective either in DNA synthesis or in virus assembly. Foreign gene expression was affected by the nature of the ts lesion and by the nature of the vaccinia promoter positioned upstream from the foreign gene. With both vector viruses, a foreign gene controlled by the p7.5 early-late promoter was expressed at both 33 degrees and 40 degrees C. With the DNA synthesis-defective vector virus, foreign gene expression controlled by the p11 DNA synthesis-dependent late promoter was inhibited at 40 degrees C, but could be turned on by shift to 33 degrees C. This ts expression system provides an alternative to use of drugs that inhibit DNA synthesis as a means for experimental manipulation of gene expression. Both vector viruses can be used with existing vaccinia virus expression technology.