Strain engineering to reduce acetate accumulation during microaerobic growth conditions in Escherichia coli.

Microaerobic (oxygen limited) conditions are advantageous for several industrial applications since a majority of the carbon atoms can be directed for synthesis of desired products. Oxygen limited conditions, however, can result in high levels of undesirable by-products such as acetate, which subsequently can have an impact on biomass and product yields. The molecular mechanisms involved in acetate accumulation under oxygen limited conditions are not well understood. Our results indicate that a majority of the genetic modifications known to decrease acetate under aerobic conditions results in similar or even higher acetate under oxygen limitation. Deletion of arcA, whose gene product is a global transcriptional regulator, was the only modification among those evaluated that significantly decreased acetate under both transient and prolonged oxygen limitation. Transcriptome results indicate that the arcA deletion results in an increased expression of the operon involving acs and actP (whose gene products are involved in acetate assimilation and uptake respectively) and some genes in the TCA cycle, thereby promoting increased acetate assimilation. These results provide useful cues for strain design for improved manufacturing of biopharmaceuticals under oxygen limited conditions. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 34:303-314, 2018.

Remote control of myosin and kinesin motors using light-activated gearshifting.

Cytoskeletal motors perform critical force generation and transport functions in eukaryotic cells. Engineered modifications of motor function provide direct tests of protein structure-function relationships and potential tools for controlling cellular processes or for harnessing molecular transport in artificial systems. Here, we report the design and characterization of a panel of cytoskeletal motors that reversibly change gears--speed up, slow down or switch directions--when exposed to blue light. Our genetically encoded structural designs incorporate a photoactive protein domain to enable light-dependent conformational changes in an engineered lever arm. Using in vitro motility assays, we demonstrate robust spatiotemporal control over motor function and characterize the kinetics of the optical gearshifting mechanism. We have used a modular approach to create optical gearshifting motors for both actin-based and microtubule-based transport.

Engineering myosins for long-range transport on actin filaments.

Cytoskeletal motors act as cargo transporters in cells and may be harnessed for directed transport applications in molecular detection and diagnostic devices. High processivity, the ability to take many steps along a track before dissociating, is often a desirable characteristic because it allows nanoscale motors to transport cargoes over distances on the scale of micrometres, in vivo and in vitro. Natural processive myosins are dimeric and use internal tension to coordinate the detachment cycles of the two heads. Here, we show that processivity can be enhanced in engineered myosins using two non-natural strategies designed to optimize the effectiveness of random, uncoordinated stepping: (1) the formation of three-headed and four-headed myosins and (2) the introduction of flexible elements between heads. We quantify improvements using systematic single-molecule characterization of a panel of engineered motors. To test the modularity of our approach, we design a controllably bidirectional myosin that is robustly processive in both forward and backward directions, and also produce the fastest processive cytoskeletal motor measured so far, reaching a speed of 10 µm s(-1).

Engineering controllable bidirectional molecular motors based on myosin.

Cytoskeletal motors drive the transport of organelles and molecular cargoes within cells and have potential applications in molecular detection and diagnostic devices. Engineering molecular motors with controllable properties will allow selective perturbation of mechanical processes in living cells and provide optimized device components for tasks such as molecular sorting and directed assembly. Biological motors have previously been modified by introducing activation/deactivation switches that respond to metal ions and other signals. Here, we show that myosin motors can be engineered to reversibly change their direction of motion in response to a calcium signal. Building on previous protein engineering studies and guided by a structural model for the redirected power stroke of myosin VI, we have constructed bidirectional myosins through the rigid recombination of structural modules. The performance of the motors was confirmed using gliding filament assays and single fluorophore tracking. Our strategy, in which external signals trigger changes in the geometry and mechanics of myosin lever arms, should make it possible to achieve spatiotemporal control over a range of motor properties including processivity, stride size and branchpoint turning.