Fig. 1

Mutant selections for Big Blue rats. a lacI selection. When LacI, the repressor protein of the lac operon, is active, it represses the expression of beta-galactosidase, which leads to colorless plaques. When the lacI gene is inactivated by mutations, beta-galactosidase is expressed, which leads to blue plaques. b cII selection. The cII protein is the critical switch in the lytic/lysogenic cycles of lambda phage. It activates the expression of the lambda cI (repressor) and int (integrase) genes, which are required for the establishment of lysogeny. The cII protein is negatively regulated by host E. coli Hfl protease, which digests the cII protein. In the hfl ^- background, the cII level is high, and therefore the lambda becomes lysogen. Only cII mutants can enter a lytic cycle and make plaques at 24 °C. The cI ^- mutants can’t enter the lytic cycle at this temperature. Therefore, the cII selection for Big Blue rats is conducted at 24 °C