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Controlling orthogonal ribosome subunit interactions enables evolution of new function.

ABSTRACT: Orthogonal ribosomes are unnatural ribosomes that are directed towards orthogonal messenger RNAs in Escherichia coli, through an altered version of the 16S ribosomal RNA of the small subunit1. Directed evolution of orthogonal ribosomes has provided access to new ribosomal function, and the evolved orthogonal ribosomes have enabled the encoding of multiple non-canonical amino acids into proteins2-4. The original orthogonal ribosomes shared the pool of 23S ribosomal RNAs, contained in the large subunit, with endogenous ribosomes. Selectively directing a new 23S rRNA to an orthogonal mRNA, by controlling the association between the orthogonal 16S rRNAs and 23S rRNAs, would enable the evolution of new function in the large subunit. Previous work covalently linked orthogonal 16S rRNA and a circularly permuted 23S rRNA to create orthogonal ribosomes with low activity5,6; however, the linked subunits in these ribosomes do not associate specifically with each other, and mediate translation by associating with endogenous subunits. Here we discover engineered orthogonal 'stapled' ribosomes (with subunits linked through an optimized RNA staple) with activities comparable to that of the parent orthogonal ribosome; they minimize association with endogenous subunits and mediate translation of orthogonal mRNAs through the association of stapled subunits. We evolve cells with genomically encoded stapled ribosomes as the sole ribosomes, which support cellular growth at similar rates to natural ribosomes. Moreover, we visualize the engineered stapled ribosome structure by cryo-electron microscopy at 3.0 Å, revealing how the staple links the subunits and controls their association. We demonstrate the utility of controlling subunit association by evolving orthogonal stapled ribosomes which efficiently polymerize a sequence of monomers that the natural ribosome is intrinsically unable to translate. Our work provides a foundation for evolving the rRNA of the entire orthogonal ribosome for the encoded cellular synthesis of non-canonical biological polymers7.

SUBMITTER: Schmied WH

PROVIDER: S-EPMC6525102 | biostudies-literature | 2018 Dec

REPOSITORIES: biostudies-literature

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Project description:When nutrients become scarce, bacteria can enter an extended state of quiescence. A major challenge of this state is how to preserve ribosomes for the return to favorable conditions. Here, we show that the ribosome dimerization protein hibernation-promoting factor (HPF) functions to protect essential ribosomal proteins. Ribosomes isolated from strains lacking HPF (Δhpf) or encoding a mutant allele of HPF that binds the ribosome but does not mediate dimerization were substantially depleted of the small subunit proteins S2 and S3. Strikingly, these proteins are located directly at the ribosome dimer interface. We used single-particle cryo-electron microscopy (cryo-EM) to further characterize these ribosomes and observed that a high percentage of ribosomes were missing S2, S3, or both. These data support a model in which the ribosome dimerization activity of HPF evolved to protect labile proteins that are essential for ribosome function. HPF is almost universally conserved in bacteria, and HPF deletions in diverse species exhibit decreased viability during starvation. Our data provide mechanistic insight into this phenotype and establish a mechanism for how HPF protects ribosomes during quiescence.IMPORTANCE The formation of ribosome dimers during periods of dormancy is widespread among bacteria. Dimerization is typically mediated by a single protein, hibernation-promoting factor (HPF). Bacteria lacking HPF exhibit strong defects in viability and pathogenesis and, in some species, extreme loss of rRNA. The mechanistic basis of these phenotypes has not been determined. Here, we report that HPF from the Gram-positive bacterium Bacillus subtilis preserves ribosomes by preventing the loss of essential ribosomal proteins at the dimer interface. This protection may explain phenotypes associated with the loss of HPF, since ribosome protection would aid survival during nutrient limitation and impart a strong selective advantage when the bacterial cell rapidly reinitiates growth in the presence of sufficient nutrients.

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Controlling orthogonal ribosome subunit interactions enables evolution of new function.

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