An inducible AraC that responds to blue light instead of arabinose

In Escherichia coli , the operon responsible for the catabolism of L-arabinose is regulated by the dimeric DNA-binding protein AraC. In the absence of L-arabinose, AraC binds to the distal I1 and O2 half-sites, leading to repression of the downstream PBAD promoter. In the presence of the sugar, the dimer changes conformation and binds to the adjacent I1 and I2 half-sites, resulting in the activation of PBAD. Here we engineer blue light-inducible AraC dimers in Escherichia coli (BLADE) by swapping the dimerization domain of AraC with blue light-inducible dimerization domains. Using BLADE to overexpress proteins important for cell shape and division site selection, we reversibly control cell morphology with light. We demonstrate the exquisite light responsiveness of BLADE by employing it to create bacteriographs with an unprecedented quality. We then employ it to perform a medium-throughput characterization of 39 E. coli genes with poorly defined or completely unknown function. Finally, we expand the initial library and create a whole family of BLADE transcription factors (TFs), which we characterize using a novel 96-well light induction setup. Since the PBAD promoter is commonly used by microbiologists, we envisage that the BLADE TFs will bring the many advantages of optogenetic gene expression to the field of microbiology. ### Competing Interest Statement The authors have declared no competing interest.