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Ribosome assembly in eukaryotes requires approximately 200 essential assembly factors (AFs), and occurs via ordered events that initiate in the nucleolus and culminate in the cytoplasm. Here we present the cryo-electron microscopy (cryo-EM) structure of a late cytoplasmic 40S ribosome assembly intermediate from Saccharomyces cerevisiae. The positions of bound AFs were defined using cryo-EM reconstructions of pre-ribosomal complexes lacking individual components. All seven AFs are positioned to prevent each step in the translation initiation pathway by obstructing the binding sites for initiation factors, by preventing the opening of the mRNA channel, by blocking 60S subunit joining, and by disrupting the decoding site. We suggest that these highly redundant mechanisms ensure that pre-40S particles do not enter the translation pathway, which would result in their rapid degradation. Implications for the regulation of 40S maturation are also discussed.

Bacteria have been extensively utilized for bioimaging, diagnosis and therapy given their unique characteristics including genetic manipulation, rapid proliferation and disease site targeting specificity. However, clinical translation of bacteria for these applications has been largely restricted by their unavoidable side effects and low treatment efficacies. Engineered bacteria for biomedical applications ideally need to generate only a low inflammatory response, show slow elimination by macrophages, low accumulation in normal organs, and almost unchanged inherent bioactivities. Here we describe a set of stealth bacteria, cell membrane coated bacteria (CMCB), meeting these requirement. Our findings are supported by evaluation in multiple mice models and ultimately demonstrate the potential of CMCB to serve as efficient tumor imaging agents. Stealth bacteria wrapped up with cell membranes have the potential for a myriad of bacterial-mediated biomedical applications.