Fundamental biological processes including morphogenesis, tissue repair and tumour metastasis require collective cell motions 1-3 , and to drive these motions cells exert traction forces on their surroundings 4 . Current understanding emphasizes that these traction forces arise mainly in 'leader cells' at the front edge of the advancing cell sheet 5-9 . Our data are contrary to that assumption and show for the first time by direct measurement that traction forces driving collective cell migration arise predominately many cell rows behind the leading front edge and extend across enormous distances. Traction fluctuations are anomalous, moreover, exhibiting broad non-Gaussian distributions characterized by exponential tails 10-12 . Taken together, these unexpected findings demonstrate that although the leader cell may have a pivotal role in local cell guidance, physical forces that it generates are but a small part of a global tug-of-war involving cells well back from the leading edge.The single adherent cell moves by the action of two synchronized cycles, one involving extension and contraction of its cytoskeleton and the other involving formation and detachment of its adhesions 13,14 . Although this complex process remains a matter of intense research [14][15][16]

Fluorescence provides a mechanism for achieving contrast in biological imaging that enables investigations of molecular structure, dynamics, and function at high spatial and temporal resolution. Small-molecule organic fluorophores have proven essential for such efforts and are widely used in advanced applications such as single-molecule and super-resolution microscopy. Yet, organic fluorophores, like all fluorescent species, exhibit instabilities in their emission characteristics, including blinking and photobleaching that limit their utility and performance. Here, we review the photophysics and photochemistry of organic fluorophores as they pertain to mitigating such instabilities, with a specific focus on the development of stabilized fluorophores through derivatization. Self-healing organic fluorophores, wherein the triplet state is intramolecularly quenched by a covalently attached protective agent, exhibit markedly improved photostabilities. We discuss the potential for further enhancements towards the goal of developing “ultra-stable” fluorophores spanning the visible spectrum and how such fluorophores are likely to impact the future of single-molecule research.

Directional translocation of the ribosome through the messenger RNA open reading frame is a critical determinant of translational fidelity. This process entails a complex interplay of large-scale conformational changes within the actively translating particle, which together coordinate the movement of transfer and messenger RNA substrates with respect to the large and small ribosomal subunits. Using pre-steady state, single-molecule fluorescence resonance energy transfer imaging, we have tracked the nature and timing of these conformational events within the Escherichia coli ribosome from five structural perspectives. Our investigations reveal direct evidence of structurally and kinetically distinct, late intermediates during substrate movement, whose resolution is rate-determining to the translocation mechanism. These steps involve intra-molecular events within the EFG(GDP)-bound ribosome, including exaggerated, reversible fluctuations of the small subunit head domain, which ultimately facilitate peptidyl-tRNA’s movement into its final post-translocation position.

Kraljic is widely viewed as a driving force behind the concepts of supply management and purchasing portfolios. Kraljic proposed that supply management professionals needed to engage in a new approach, embracing globalization, technology and risk. This article marked a critical juncture for supply chain management. Almost three decades later, it is evident that the purchasing portfolio concept has been widely adopted as an effective practitioner tool and a well‐accepted tenet in the supply chain management literature. However, a recently completed study yielded interesting evidence of a potential shift in supply chain management, specifically, in sustainable sourcing. We recently observed that a number of leaders in sustainable supply chain management (SSCM) were not organizing purchasing portfolios in the manner suggested by Kraljic. We found organizations buying what would traditionally be leveraged commodities in a manner more appropriate for strategic suppliers. This unexpected observation suggests that the supply chain field may face another critical juncture, this time related to SSCM. This manuscript describes the observed phenomena and then, using an inductive approach, enhances the existing theory to explain what was observed. The end result is a modified sustainable purchasing portfolio model that should provide a strategic tool to help both academics and practitioners adapt to the new realities of SSCM.

Molecular recognition is often driven by transient processes beyond the reach of detection. Single-molecule fluorescence microscopy methods are uniquely suited for detecting such non-accumulating intermediates, yet achieving the time resolution and statistics to realize this potential has proven challenging. Here, we present a single-molecule fluorescence resonance energy transfer (smFRET) imaging and analysis platform leveraging advances in scientific complementary metal-oxide semiconductor (sCMOS) detectors that enable the imaging of more than 10,000 individual molecules simultaneously at millisecond rates. The utility of this advance is demonstrated through quantitative measurements of previously obscured processes relevant to the fidelity mechanism in protein synthesis.

Protein synthesis by the ribosome is highly dependent on the ionic conditions in the cellular environment, but the roles of ribosome solvation remain poorly understood. Moreover, the function of modifications to ribosomal RNA and ribosomal proteins are unclear. Here we present the structure of the Escherichia coli 70S ribosome to 2.4 Å resolution. The structure reveals details of the ribosomal subunit interface that are conserved in all domains of life, and suggest how solvation contributes to ribosome integrity and function. The structure also suggests how the conformation of ribosomal protein uS12 likely impacts its contribution to messenger RNA decoding. This structure helps to explain the phylogenetic conservation of key elements of the ribosome, including posttranscriptional and posttranslational modifications and should serve as a basis for future antibiotic development.

Protein synthesis is targeted by numerous, chemically distinct antibiotics that bind and inhibit key functional centers of the ribosome. Using single-molecule imaging and X-ray crystallography, we show that the aminoglycoside neomycin blocks aminoacyl–transfer RNA (aa-tRNA) selection and translocation as well as ribosome recycling by binding to helix 69 (H69) of 23S ribosomal RNA within the large subunit of the Escherichia coli ribosome. There, neomycin prevents the remodeling of intersubunit bridges that normally accompanies the process of subunit rotation to stabilize a partially rotated ribosome configuration in which peptidyl (P)-site tRNA is constrained in a previously unidentified hybrid position. Direct measurements show that this neomycin-stabilized intermediate is incompatible with the translation factor binding that is required for distinct protein synthesis reactions. These findings reveal the functional importance of reversible intersubunit rotation to the translation mechanism and shed new light on the allosteric control of ribosome functions by small-molecule antibiotics.

In bacteria, the translocation of tRNA and mRNA with respect to the ribosome is catalyzed by the conserved GTPase, elongation factor-G (EF-G). In order to probe the rate determining features in this process, EF-G-catalyzed translocation was imaged from two unique structural perspectives using single-molecule fluorescence resonance energy transfer. The data reveal that the rate at which the ribosome spontaneously achieves a transient, “unlocked” state is closely correlated with the rate at which the tRNA-like, domain IV/V element of EF-G engages the A site. Following these structural transitions, translocation occurs comparatively fast, suggesting that conformational processes intrinsic to the ribosome determine the rate of translocation. Experiments performed in the presence of non-hydrolyzable GTP analogues and specific antibiotics further reveal that allosterically linked conformational events in EF-G and the ribosome mediate rapid, directional substrate movement and EF-G release.