The antibiotic linezolid, the first clinically approved member of the oxazolidinone class, inhibits translation of bacterial ribosomes by binding to the peptidyl transferase center. Recent work has demonstrated that linezolid does not inhibit peptide bond formation at all sequences but rather acts in a context-specific manner, namely when alanine occupies the penultimate position of the nascent chain. In this study, we determined that the second-generation oxazolidinone radezolid also induces stalling with alanine at the penultimate position. However, the molecular basis for context-specificity of these inhibitors has not been elucidated. In this study, we determined high-resolution cryo-EM structures of both linezolid and radezolid-stalled ribosome complexes. These structures reveal that the alanine side chain fits within a small hydrophobic crevice created by oxazolidinone, resulting in improved ribosome binding. Modification of the ribosome by the antibiotic resistance enzyme Cfr disrupts stalling by forcing the antibiotic to adopt a conformation that narrows the hydrophobic alanine pocket. Together, the structural and biochemical findings presented in this work provide molecular understanding of context-specific inhibition of translation by clinically important oxazolidinone antibiotics.

Chemical probes for chromatin reader proteins are valuable tools for investigating epigenetic regulatory mechanisms and evaluating whether the target of interest holds therapeutic potential. Developing potent inhibitors for plant homeodomain...

PHD reader domains are chromatin binding modules often responsible for the recruitment of large protein complexes that contain histone modifying enzymes, chromatin remodelers and DNA repair machinery. PHD domains recognize N-terminal residues of histone H3 and are sensitive to the methylation state of Lys4 in histone H3 (H3K4). Histone demethylase KDM5A, an epigenetic eraser enzyme that contains three PHD domains, is often overexpressed in various cancers and its demethylation activity is allosterically enhanced when its PHD1 domain is bound to the H3 tail. The allosteric regulatory function of PHD1 expands roles of reader domains, suggesting unique features of this chromatin interacting module. Our previous studies determined the H3 binding site of PHD1, although it remains unclear how the H3 tail interacts with the N-terminal residues of PHD1 and how PHD1 discriminates against H3 tails with varying degrees of H3K4 methylation. Here we show that the N-terminal region of PHD1 is initially unstructured, but becomes ordered upon binding of the high affinity H3 peptide ligands. We also observe differential interactions of conserved binding residues with differently methylated H3K4 peptides (me0, me1, me2 or me3), providing a rational for this PHD1 domain's preference for lower methylation states of H3K4.We further assessed the contributions of various H3 interacting residues in the PHD1 domain to the binding of H3 peptides using mutagenesis. The insights into the H3 binding site could provide useful information to aid development of allosteric small molecule modulators of KDM5A.