Active vaccinia virus topoisomerase I is expressed in Escherichia coli containing plasmid p1940 (S. Shuman, M. Golder, and B. Moss, J. Biol. Chem. 263:16401-16407, 1988). Growth curves showed a decline of 2 to 3 logs in the number of viable cells at 42-degrees-C after shift from 30-degrees-C because of increased vaccinia virus topoisomerase I level. Mutations in the gyrA and gyrB genes allowed cells to grow equally well at 42 and 30-degrees-C. The presence of gyrase inhibitor also improved growth at 42-degrees-C.