756 research outputs found
Microwave Gaseous Discharges
Contains reports on three research projects.Atomic Energy Commission under Contract AT(30-1)- 184
Epigenetic Chromatin Silencing: Bistability and Front Propagation
The role of post-translational modification of histones in eukaryotic gene
regulation is well recognized. Epigenetic silencing of genes via heritable
chromatin modifications plays a major role in cell fate specification in higher
organisms. We formulate a coarse-grained model of chromatin silencing in yeast
and study the conditions under which the system becomes bistable, allowing for
different epigenetic states. We also study the dynamics of the boundary between
the two locally stable states of chromatin: silenced and unsilenced. The model
could be of use in guiding the discussion on chromatin silencing in general. In
the context of silencing in budding yeast, it helps us understand the phenotype
of various mutants, some of which may be non-trivial to see without the help of
a mathematical model. One such example is a mutation that reduces the rate of
background acetylation of particular histone side-chains that competes with the
deacetylation by Sir2p. The resulting negative feedback due to a Sir protein
depletion effect gives rise to interesting counter-intuitive consequences. Our
mathematical analysis brings forth the different dynamical behaviors possible
within the same molecular model and guides the formulation of more refined
hypotheses that could be addressed experimentally.Comment: 19 pages, 5 figure
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Reactive Multiphase Behavior of Co{sub 2} in Saline Aquifers Beneath the Colorado Plateau
Soil gas surveys have been carried out on the Colorado Plateau over areas with natural occurrences of CO{sub 2}. At Farnham Dome, Utah, and Springerville-St. Johns, Arizona, proven CO{sub 2} reservoirs occur at 600-800 m depth, but no anomalous soil gas CO{sub 2} flux was detected. Background CO{sub 2} fluxes of up to about 5 g m{sup -2} day{sup -1} were common in arid, poorly vegetated areas, and fluxes up to about 20 g m{sup -2} day{sup -1} were found at Springerville-St. Johns in heavily vegetated, wet ground adjacent to springs. These elevated fluxes are attributed to shallow root zone activity rather than to a deep upflow of CO{sub 2}. Localized areas of anomalously high CO{sub 2} gas flux ({approx} 100 g m{sup -2} day{sup -1}) were documented along the Little Grand Wash Fault Zone near Crystal Geyser, Utah and nearby in Ten Mile Graben, but those in Ten Mile Graben are not directly associated with the major faults. In both areas, features with a visible gas flux are present. Isotopic measurements on the CO{sub 2} gas confirm that it originated at depth. Evidence of widespread vein calcite at the surface at Farnham Dome and travertine deposits in the other areas suggests that there has been an outflow of CO{sub 2}-rich fluids in the past. 14C ages of pollen trapped in the travertine at Springerville-St. Johns record a period of CO{sub 2} leakage to the atmosphere between 887 {+-} 35 and 3219 {+-} 30 years BP. No travertine deposits appear to be currently forming. At Springerville-St. Johns, Crystal Geyser and Ten Mile Graben, there are significant outflows of high-bicarbonate water. Movement of CO{sub 2}-rich groundwaters may be the dominant mechanism controlling the mobility of CO{sub 2} today. The very localized nature of the soil gas anomalies, evidence of large scale discharge of CO{sub 2} over a very short period of time and the outflow of ground water containing dissolved CO{sub 2} will present challenges for effective, long term monitoring of CO{sub 2} leakage
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