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June 28, 2016

Amino acid sequence repertoire of the bacterial proteome and the occurrence of untranslatable sequences.

Bioinformatic analysis of Escherichia coli proteomes revealed that all possible amino acid triplet sequences occur at their expected frequencies, with four exceptions. Two of the four underrepresented sequences (URSs) were shown to interfere with translation in vivo and in vitro. Enlarging the URS by a single amino acid resulted in increased translational inhibition. Single-molecule methods revealed stalling of translation at the entrance of the peptide exit tunnel of the ribosome, adjacent to ribosomal nucleotides A2062 and U2585. Interaction with these same ribosomal residues is involved in regulation of translation by longer, naturally occurring protein sequences. The E. coli exit tunnel…

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April 7, 2016

Multiple parallel pathways of translation initiation on the CrPV IRES.

The complexity of eukaryotic translation allows fine-tuned regulation of protein synthesis. Viruses use internal ribosome entry sites (IRESs) to minimize or, like the CrPV IRES, eliminate the need for initiation factors. Here, by exploiting the CrPV IRES, we observed the entire process of initiation and transition to elongation in real time. We directly tracked the CrPV IRES, 40S and 60S ribosomal subunits, and tRNA using single-molecule fluorescence spectroscopy and identified multiple parallel initiation pathways within the system. Our results distinguished two pathways of 80S:CrPV IRES complex assembly that produce elongation-competent complexes. Following 80S assembly, the requisite eEF2-mediated translocation results in…

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March 13, 2016

Probing the translation dynamics of ribosomes using Zero-Mode Waveguides

In order to coordinate the complex biochemical and structural feat of converting triple-nucleotide codons into their corresponding amino acids, the ribosome must physically manipulate numerous macromolecules including the mRNA, tRNAs, and numerous translation factors. The ribosome choreographs binding, dissociation, physical movements, and structural rearrangements so that they synergistically harness the energy from biochemical processes, including numerous GTP hydrolysis steps and peptide bond formation. Due to the dynamic and complex nature of translation, the large cast of ligands involved, and the large number of possible configurations, tracking the global time evolution or dynamics of the ribosome complex in translation has proven…

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March 5, 2016

Genomic and transcriptomic analysis of the streptomycin-dependent Mycobacterium tuberculosis strain 18b.

The ability of Mycobacterium tuberculosis to establish a latent infection (LTBI) in humans confounds the treatment of tuberculosis. Consequently, there is a need to discover new therapeutic agents that can kill M. tuberculosis both during active disease and LTBI. The streptomycin-dependent strain of M. tuberculosis, 18b, provides a useful tool for this purpose since upon removal of streptomycin (STR) it enters a non-replicating state that mimics latency both in vitro and in animal models.The 4.41 Mb genome sequence of M. tuberculosis 18b was determined and this revealed the strain to belong to clade 3 of the ancient ancestral lineage of the…

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February 22, 2016

Genome sequence and analysis of Escherichia coli MRE600, a colicinogenic, nonmotile strain that lacks RNase I and the type I methyltransferase, EcoKI.

Escherichia coli strain MRE600 was originally identified for its low RNase I activity and has therefore been widely adopted by the biomedical research community as a preferred source for the expression and purification of transfer RNAs and ribosomes. Despite its widespread use, surprisingly little information about its genome or genetic content exists. Here, we present the first de novo assembly and description of the MRE600 genome and epigenome. To provide context to these studies of MRE600, we include comparative analyses with E. coli K-12 MG1655 (K12). Pacific Biosciences Single Molecule, Real-Time sequencing reads were assembled into one large chromosome (4.83…

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November 19, 2015

Coupling of mRNA structure rearrangement to ribosome movement during bypassing of non-coding regions.

Nearly half of the ribosomes translating a particular bacteriophage T4 mRNA bypass a region of 50 nt, resuming translation 3' of this gap. How this large-scale, specific hop occurs and what determines whether a ribosome bypasses remain unclear. We apply single-molecule fluorescence with zero-mode waveguides to track individual Escherichia coli ribosomes during translation of T4's gene 60 mRNA. Ribosomes that bypass are characterized by a 10- to 20-fold longer pause in a non-canonical rotated state at the take-off codon. During the pause, mRNA secondary structure rearrangements are coupled to ribosome forward movement, facilitated by nascent peptide interactions that disengage the…

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October 13, 2015

Mistranslation drives the evolution of robustness in TEM-1 ß-lactamase.

How biological systems such as proteins achieve robustness to ubiquitous perturbations is a fundamental biological question. Such perturbations include errors that introduce phenotypic mutations into nascent proteins during the translation of mRNA. These errors are remarkably frequent. They are also costly, because they reduce protein stability and help create toxic misfolded proteins. Adaptive evolution might reduce these costs of protein mistranslation by two principal mechanisms. The first increases the accuracy of translation via synonymous "high fidelity" codons at especially sensitive sites. The second increases the robustness of proteins to phenotypic errors via amino acids that increase protein stability. To study…

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September 15, 2015

Cotranslational protein folding inside the ribosome exit tunnel.

At what point during translation do proteins fold? It is well established that proteins can fold cotranslationally outside the ribosome exit tunnel, whereas studies of folding inside the exit tunnel have so far detected only the formation of helical secondary structure and collapsed or partially structured folding intermediates. Here, using a combination of cotranslational nascent chain force measurements, inter-subunit fluorescence resonance energy transfer studies on single translating ribosomes, molecular dynamics simulations, and cryoelectron microscopy, we show that a small zinc-finger domain protein can fold deep inside the vestibule of the ribosome exit tunnel. Thus, for small protein domains, the ribosome itself can provide…

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June 1, 2014

Dynamic pathways of -1 translational frameshifting.

Spontaneous changes in the reading frame of translation are rare (frequency of 10(-3) to 10(-4) per codon), but can be induced by specific features in the messenger RNA (mRNA). In the presence of mRNA secondary structures, a heptanucleotide 'slippery sequence' usually defined by the motif X XXY YYZ, and (in some prokaryotic cases) mRNA sequences that base pair with the 3' end of the 16S ribosomal rRNA (internal Shine-Dalgarno sequences), there is an increased probability that a specific programmed change of frame occurs, wherein the ribosome shifts one nucleotide backwards into an overlapping reading frame (-1 frame) and continues by…

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May 1, 2014

The dynamics of SecM-induced translational stalling.

SecM is an E. coli secretion monitor capable of stalling translation on the prokaryotic ribosome without cofactors. Biochemical and structural studies have demonstrated that the SecM nascent chain interacts with the 50S subunit exit tunnel to inhibit peptide bond formation. However, the timescales and pathways of stalling on an mRNA remain undefined. To provide a dynamic mechanism for stalling, we directly tracked the dynamics of elongation on ribosomes translating the SecM stall sequence (FSTPVWISQAQGIRAGP) using single-molecule fluorescence techniques. Within 1 min, three peptide-ribosome interactions work cooperatively over the last five codons of the SecM sequence, leading to severely impaired elongation rates beginning from…

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May 1, 2014

Sequence-dependent elongation dynamics on macrolide-bound ribosomes.

The traditional view of macrolide antibiotics as plugs inside the ribosomal nascent peptide exit tunnel (NPET) has lately been challenged in favor of a more complex, heterogeneous mechanism, where drug-peptide interactions determine the fate of a translating ribosome. To investigate these highly dynamic processes, we applied single-molecule tracking of elongating ribosomes during inhibition of elongation by erythromycin of several nascent chains, including ErmCL and H-NS, which were shown to be, respectively, sensitive and resistant to erythromycin. Peptide sequence-specific changes were observed in translation elongation dynamics in the presence of a macrolide-obstructed NPET. Elongation rates were not severely inhibited in general…

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February 21, 2013

The impact of aminoglycosides on the dynamics of translation elongation.

Inferring antibiotic mechanisms on translation through static structures has been challenging, as biological systems are highly dynamic. Dynamic single-molecule methods are also limited to few simultaneously measurable parameters. We have circumvented these limitations with a multifaceted approach to investigate three structurally distinct aminoglycosides that bind to the aminoacyl-transfer RNA site (A site) in the prokaryotic 30S ribosomal subunit: apramycin, paromomycin, and gentamicin. Using several single-molecule fluorescence measurements combined with structural and biochemical techniques, we observed distinct changes to translational dynamics for each aminoglycoside. While all three drugs effectively inhibit translation elongation, their actions are structurally and mechanistically distinct. Apramycin does…

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June 1, 2012

Heterogeneous pathways and timing of factor departure during translation initiation.

The initiation of translation establishes the reading frame for protein synthesis and is a key point of regulation. Initiation involves factor-driven assembly at a start codon of a messenger RNA of an elongation-competent 70S ribosomal particle (in bacteria) from separated 30S and 50S subunits and initiator transfer RNA. Here we establish in Escherichia coli, using direct single-molecule tracking, the timing of initiator tRNA, initiation factor 2 (IF2; encoded by infB) and 50S subunit joining during initiation. Our results show multiple pathways to initiation, with orders of arrival of tRNA and IF2 dependent on factor concentration and composition. IF2 accelerates 50S…

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April 15, 2010

Real-time tRNA transit on single translating ribosomes at codon resolution.

Translation by the ribosome occurs by a complex mechanism involving the coordinated interaction of multiple nucleic acid and protein ligands. Here we use zero-mode waveguides (ZMWs) and sophisticated detection instrumentation to allow real-time observation of translation at physiologically relevant micromolar ligand concentrations. Translation at each codon is monitored by stable binding of transfer RNAs (tRNAs)-labelled with distinct fluorophores-to translating ribosomes, which allows direct detection of the identity of tRNA molecules bound to the ribosome and therefore the underlying messenger RNA (mRNA) sequence. We observe the transit of tRNAs on single translating ribosomes and determine the number of tRNA molecules simultaneously…

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