Single-cell visual proteomics of a minimal bacterium reveals structural coordination of gene expression machineries

Translation is a central process in gene expression. Its regulation is complex, depends on factors that include cell state and the subcellular environment, and is subject to modulation via crosstalk to processes such as transcription or translocation. Here, we used cryo-electron tomography of native and antibiotic-perturbed Mycoplasma pneumoniae cells to resolve 140 maps that recapitulate bacterial translation during the initiation, elongation, and recycling phases. We visualized multiple transcription-translation complexes, allowing us to propose a threading-based translation reinitiation mechanism and to provide structural evidence for a long-hypothesized supercomplex that coordinates transcription, translation, and membrane attachment. We resolved abundant membrane-associated large ribosomal subunits and suggest that dissociation from membranes depends on the conditional initiation of new translation, consistent with a potentially conserved mechanism in mammalian cells. This work visualizes the multilayered control of bacterial translation and demonstrates the power of in-cell structural biology to investigate regulatory circuits in gene expression.