Antibiotic production and resistance pathways in Streptomyces are dictated by the interplay of transcriptional regulatory proteins that trigger downstream responses via binding to small diffusible molecules. To decipher the mode of DNA binding and the associated allosteric mechanism in the sub-class of transcription factors that are induced by γ-butyrolactones, we present the crystal structure of CprB in complex with the consensus DNA element to a resolution of 3.25 Å. Binding of the DNA results in the restructuring of the dimeric interface of CprB, inducing a pendulum-like motion of the helix-turn-helix motif that inserts into the major groove. The crystal structure revealed that, CprB is bound to DNA as a dimer of dimers with the mode of binding being analogous to the broad spectrum multidrug transporter protein QacR from the antibiotic resistant strain Staphylococcus aureus. It was demonstrated that the CprB displays a cooperative mode of DNA binding, following a clamp and click model. Experiments performed on a subset of DNA sequences from Streptomyces coelicolor A3(2) suggest that CprB is most likely a pleiotropic regulator. Apart from serving as an autoregulator, it is potentially a part of a network of proteins that modulates the γ-butyrolactone synthesis and antibiotic regulation pathways in S. coelicolor A3(2).

Post-translational methylation of rRNA at select positions is a prevalent resistance mechanism adopted by pathogens. In this work, KsgA, a housekeeping ribosomal methyltransferase (rMtase) involved in ribosome biogenesis, was exploited as a model system to delineate the specific targeting determinants that impart substrate specificity to rMtases. With a combination of evolutionary and structure-guided approaches, a set of chimeras were created that altered the targeting specificity of KsgA such that it acted similarly to erythromycin-resistant methyltransferases (Erms), rMtases found in multidrug-resistant pathogens. The results revealed that specific loop embellishments on the basic Rossmann fold are key determinants in the selection of the cognate RNA. Moreover, in vivo studies confirmed that chimeric constructs are competent in imparting macrolide resistance. This work explores the factors that govern the emergence of resistance and paves the way for the design of specific inhibitors useful in reversing antibiotic resistance.

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