29 Mutation Research, 50 (1978) 29--35 © Elsevier/North-Holland Biomedical Press EXPRESSION OF A BACTERIAL GENE TURNED ON BY A POTENT CARCINOGEN A. LEVINE, P.L. MOREAU, S.G. SEDGWICK and R. DEVORET Section de Radiobiologie Cellulaire, Laboratoire d 'Enzymologie, C.N.R.S., 91190 Gif-sur-Yvette (France) and S. ADHYA, M. GOTTESMAN and A. DAS National Cancer Institute, National Institutes of Health, Bethesda, Md. 20014 (U.S.A.) (Received 26 August 1977) (Accepted 31 October 1977) Summary Contrary to mutagenesis, lysogenic induction produced by chemical carcinogens occurs in the majority of a population of lysogenic cells. Such a mass effect can therefore be measured at the biochemical level using an E. coil tester strain in which the galactose operon has been put under the negative control of the k repressor. In this publication we show that galactokinase synthesis is turned on by aflatoxin B1 metabolites within an hour after treat- ment of the tester bacteria. Such a biochemical assay provides a useful means for identifying potential chemical carcinogens. Introduction In a recent publication, Moreau et al. [19] described a test for potential carcinogens called the "Inductest" since it is based upon the induction of prophage ~ in Escherichia coli K12. In the classical techniques of mutation test- ing [4] mutagenesis is measured in at most 10 -3 cells in a population whereas in the inductest most of the treated lysogenic cells undergo a dramatic change: they lyse and release free phage. Such a mass effect should be amenable to detection in a biochemical assay. We describe here a method whereby potential carcinogens can be detected within 2 h by measuring the induction of the syn- thesis of galactokinase. We have employed a strain of E. coli K12 in which the galactose operon is integrated into an early transcribed region of prophage and is thus directly under the negative control of the ~ repressor. Induction of prophage ~ leads therefore to the synthesis of galactokinase as demonstrated with the use of aflatoxin B1, a potent hepatocarcinogen for several species probably including man [ 28].