In this paper we report the steady-state optical properties of a series of site-directed mutants in the Fenna-Matthews-Olson (FMO) complex of Chlorobaculum tepidum, a photosynthetic green sulfur ...bacterium. The FMO antenna complex has historically been used as a model system for energy transfer due to the water-soluble nature of the protein, its stability at room temperature, as well as the availability of high-resolution structural data. Eight FMO mutants were constructed with changes in the environment of each of the bacteriochlorophyll a pigments found within each monomer of the homotrimeric FMO complex. Our results reveal multiple changes in low temperature absorption, as well as room temperature CD in each mutant compared to the wild-type FMO complex. These datasets were subsequently used to model the site energies of each pigment in the FMO complex by employing three different Hamiltonians from the literature. This enabled a basic approximation of the site energy shifts imparted on each pigment by the changed amino acid residue. These simulations suggest that, while the three Hamiltonians used in this work provide good fits to the wild-type FMO absorption spectrum, further efforts are required to obtain good fits to the mutant minus wild-type absorption difference spectra. This demonstrates that the use of FMO mutants can be a valuable tool to refine and iterate the current models of energy transfer in this system.
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•The environment of each pigment in the FMO complex was perturbed by mutagenesis.•Each FMO mutant alters the optical properties of the complex in a specific manner.•Datasets obtained from the mutants were simulated with three different Hamiltonians.•Simulations of the mutant spectra fit the absorption data better than CD data.•Data from FMO mutants can be useful tools for future biophysical simulations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Previous studies of Totten Ice Shelf have employed surface velocity
measurements to estimate its mass balance and understand its sensitivities to
interannual changes in climate forcing. However, ...displacement measurements
acquired over timescales of days to weeks may not accurately characterize
long-term flow rates wherein ice velocity fluctuates with the seasons.
Quantifying annual mass budgets or analyzing interannual changes in ice
velocity requires knowing when and where observations of glacier velocity
could be aliased by subannual variability. Here, we analyze 16 years of
velocity data for Totten Ice Shelf, which we generate at subannual resolution
by applying feature-tracking algorithms to several hundred satellite image
pairs. We identify a seasonal cycle characterized by a spring to autumn
speedup of more than 100 m yr−1 close to the ice front. The amplitude
of the seasonal cycle diminishes with distance from the open ocean,
suggesting the presence of a resistive back stress at the ice front that is
strongest in winter. Springtime acceleration precedes summer surface melt and
is not attributable to thinning from basal melt. We attribute the onset of
ice shelf acceleration each spring to the loss of buttressing from the
breakup of seasonal landfast sea ice.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Using recently released reprocessed gravity solutions from the Gravity Recovery and Climate Experiment (GRACE), we estimate the ice loss rate for the Patagonia Icefield (PIF) of South America, for ...the period April 2002 through December 2006. After postglacial rebound and hydrological effects are corrected, the estimated rate is −27.9 ± 11 km3/year, equivalent to an average loss of ∼−1.6 m/year ice thickness change if evenly distributed over the entire PIF area. The estimated contribution to global sea level rise is 0.078 ± 0.031 mm/year. This is an independent confirmation of relatively large melting rate estimates from earlier studies employing topographic and cartographic data.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
This investigation assessed the extent to which elementary-age learning disabled students were able to generalize a computational skill. The subjects consisted of nine students, all of whom made ...systematic errors in arithmetic. Two experiments were conducted using within-subject designs. All students experienced baseline, demonstration and modeling plus feedback, and maintenance. Students who met criteria for generalizing proceeded to a followup condition, while those who did not received further interventions designed to increase their ability to generalize. The effectiveness of the following procedures was investigated: (a) reinforcing students "to generalize," and (b) a technique which combined training several response exemplars with the use of a verbal cue and an indiscriminable reinforcement contingency. The results indicated that demonstration and modeling plus feedback was sufficient to increase some students' ability to generalize; others required further interventions to facilitate generalization.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Despite evidence for plumes on Jupiter's moon Europa, no surface features have been definitively identified as the source of the plumes to date. Furthermore, it remains unknown whether the activity ...originates from near‐surface water reservoirs within the ice shell or if it is sourced from the underlying global ocean. Here we investigate brine pocket migration, studied previously in the context of sea ice on Earth, as a process for transporting brine along thermal gradients. We show that the fracture system located in the center of Europa's Manannán crater is consistent with the formation of a subsurface brine reservoir. After the initial impact, residual aqueous melt concentrated via brine pocket migration as the target material cooled. Freezing and overpressurization then resulted in a cryovolcanic eruption. The volume of the emptied reservoir and the critical composition at the end of migration provide further constraints on the average salinity of Europa's ice shell.
Plain Language Summary
Jupiter's satellite Europa has a subsurface ocean covered by an icy shell. We show how small pockets of brine can migrate within the ice from colder areas to warmer areas. This can happen even at very low temperatures, below the point where pure water would freeze, because the water becomes saltier and saltier as it migrates. By looking at an impact crater on Europa, which was initially warm in the center and cooled inward from its colder surroundings, we can study how the water migrated toward the center and formed a central water reservoir. As the final water pocket at the center of the crater started to freeze, the increasing pressure lead to a cryovolcanic eruption that emplaced brine onto the surface to form a prominent “spider” feature before the ice collapsed into the cavity below. Using a digital terrain model of the crater and collapse feature, we estimate how much water erupted and how salty Europa's ice shell is.
Key Points
We introduce brine pocket migration as a mechanism for melt mobilization and a driver for cryovolcanism on icy worlds
We apply brine pocket migration to Europa's Manannán crater and its central “spider” (araneiform) fracture system and collapse structure
We show that understanding the concepts of brine pocket migration can also constrain the salinity of Europa's ice shell and ocean
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Mitotic and cytokinetic processes harness cell machinery to drive chromosomal segregation and the physical separation of dividing cells. Here, we investigate the functional requirements for exocyst ...complex function during cell division in vivo, and demonstrate a common mechanism that directs anaphase cell elongation and cleavage furrow progression during cell division. We show that onion rings (onr) and funnel cakes (fun) encode the Drosophila homologs of the Exo84 and Sec8 exocyst subunits, respectively. In onr and fun mutant cells, contractile ring proteins are recruited to the equatorial region of dividing spermatocytes. However, cytokinesis is disrupted early in furrow ingression, leading to cytokinesis failure. We use high temporal and spatial resolution confocal imaging with automated computational analysis to quantitatively compare wild-type versus onr and fun mutant cells. These results demonstrate that anaphase cell elongation is grossly disrupted in cells that are compromised in exocyst complex function. Additionally, we observe that the increase in cell surface area in wild type peaks a few minutes into cytokinesis, and that onr and fun mutant cells have a greatly reduced rate of surface area growth specifically during cell division. Analysis by transmission electron microscopy reveals a massive build-up of cytoplasmic astral membrane and loss of normal Golgi architecture in onr and fun spermatocytes, suggesting that exocyst complex is required for proper vesicular trafficking through these compartments. Moreover, recruitment of the small GTPase Rab11 and the PITP Giotto to the cleavage site depends on wild-type function of the exocyst subunits Exo84 and Sec8. Finally, we show that the exocyst subunit Sec5 coimmunoprecipitates with Rab11. Our results are consistent with the exocyst complex mediating an essential, coordinated increase in cell surface area that potentiates anaphase cell elongation and cleavage furrow ingression.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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