It is often assumed that antibiotics act on the most vulnerable cellular targets, particularly those that require limited inhibition to block growth. To evaluate this assumption, we developed a genetic method that can inducibly deplete targeted proteins and that mimics their chemical inactivation. We applied this system to current antibiotic targets in mycobacteria. Although depleting some antibiotic targets significantly perturbs bacterial growth, surprisingly, we found that reducing the levels of other targets by more than 97% had little or no effect on growth. For one of these targets, dihydrofolate reductase, metabolic analysis suggested that depletion mimics the use of subinhibitory concentrations of the antibiotic trimethroprim. These observations indicate that some drug targets can exist at levels much higher than are needed to support growth. However, protein depletion can be used to identify promising drug targets that are particularly vulnerable to inhibition.
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