title: The native 3D organization of bacterial polysomes.
abstract: Recent advances have led to insights into the structure of the bacterial ribosome, but little is known about the 3D organization of ribosomes in the context of translating polysomes. We employed cryoelectron tomography and a template-matching approach to map 70S ribosomes in vitrified bacterial translation extracts and in lysates of active E. coli spheroplasts. In these preparations, polysomal arrangements were observed in which neighboring ribosomes are densely packed and exhibit preferred orientations. Analysis of characteristic examples of polysomes reveals a staggered or pseudohelical organization of ribosomes along the mRNA trace, with the transcript being sequestered on the inside, the tRNA entrance sites being accessible, and the polypeptide exit sites facing the cytosol. Modeling of elongating nascent polypeptide chains suggests that this arrangement maximizes the distance between nascent chains on adjacent ribosomes, thereby reducing the probability of intermolecular interactions that would give rise to aggregation and limit productive folding.
authors.affiliation: Department of Molecular Structural Biology, Max Planck Institute of Biochemistry, Am Klopferspitz 18, Martinsried 82152, Germany.
authors.forename: Florian
authors.initials: F
authors.lastname: Brandt
authors.forename: Stephanie A
authors.initials: SA
authors.lastname: Etchells
authors.forename: Julio O
authors.initials: JO
authors.lastname: Ortiz
authors.forename: Adrian H
authors.initials: AH
authors.lastname: Elcock
authors.forename: F Ulrich
authors.initials: FU
authors.lastname: Hartl
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Baumeister
mesh_terms: D020285:Cryoelectron Microscopy; D055032:Electron Microscope Tomography; D004926:Escherichia coli / Q000648:ultrastructure*; D008958:Models, Molecular; D011132:Polyribosomes / Q000648:ultrastructure*