A biochemical (colorimetric) assay of bacteriophage lambda induction was utilized in the detection, identification, and purification of DNA-interacting natural products with potential antitumor activity. A set of 142 standard antibiotics, composed principally of natural products with established antitumor activity and/or defined mechanisms of action, was tested in the assay. As expected, most inducers were direct inhibitors of DNA synthesis. A few other types of inducer, with probable indirect

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... ffects on DNA synthesis, were found after prolonged incubation: one class of RNA synthesis inhibitor, a dihydrofolate reductase inhibitor; and two inhibitors of bacterial cell wall synthesis. The biochemical induction assay was semiautomated for use as a prescreen in the search for novel antitumor agents in 10,724 actinomycete fermentation broths. Approximately 1% of the cultures produced compounds that were active in the assay; some appear to be novel. None required metabolic activation (via rat liver S9) for inducing activity. The biochemical induction assay was adapted for bioautography (the detection of inducing chemicals chromatographed on thin-layer plates) and for strain improvement programs (selection of isolates with enhanced inducing activity). The speed of the assay (2 to 5 hr) made it useful for monitoring antitumor agent production and purification. The sensitivity of the assay could be varied, depending on the length of the incubation period. Microbes, nutrients, and toxic solvents did not usually interfere with the detection of inducing activity.