619 research outputs found
In vivo and in vitro characterization of Staphylococcus aureus and Bacillus subtilis polyglycerolphosphate lipoteichoic acid synthases
Staphylococcus aureus lipoteichoic acid (LTA) consists of a 1,3-linked
polyglycerolphosphate chain retained in the bacterial membrane by a glycolipid
anchor. The LTA backbone is produced by the lipoteichoic acid synthase LtaS, a
membrane protein with five transmembrane helices and a large extracellular
enzymatic domain (eLtaS). Proteomic studies revealed that LtaS is efficiently cleaved,
and here it was demonstrated that the eLtaS domain is released into the culture
supernatant as well as partially retained within the cell wall fraction. However, using
an in vivo LtaS activity assay, it was shown that only the full-length LtaS enzyme is
able to synthesize LTA. Neither expression of a secreted eLtaS variant, created by
replacing the N-terminal membrane domain with a conventional signal sequence, nor
expression of eLtaS fused to a single or multi-transmembrane domains of other
staphylococcal proteins resulted in the production of LTA. These data indicate that the
transmembrane domain of LtaS play an essential, yet unknown, role in LtaS enzyme
function. In addition, the protease responsible for LtaS cleavage was identified. It was
found that a S. aureus strain in which the gene encoding for the essential signal
peptidase SpsB was cloned under inducible expression control showed an
accumulation of the full-length LtaS enzyme in the absence of the inducer. These data
suggest that SpsB is involved in LtaS cleavage.
Four LtaS orthologues, YflE, YfnI, YqgS and YvgJ, are present in Bacillus
subtilis. Using an in vitro enzyme assay and purified protein, it was determined that
all four B. subtilis proteins are Mn2+-dependent metal enzymes that use the lipid
phosphatidylglycerol as substrate. It was shown that YflE, YfnI and YqgS are bonafide
LTA synthases capable of producing polyglycerolphosphate chains, while YvgJ
appears to function as an LTA primase, as indicated by the accumulation of a
glycolipid with the expected chromatographic mobility of GroP-Glc2-DAG. Taken
together, experimental evidence for the enzyme function of all four B. subtilis LtaStype
proteins is provided in this work and it was shown that all four enzymes are
involved in the LTA synthesis process
Separable Cosparse Analysis Operator Learning
The ability of having a sparse representation for a certain class of signals
has many applications in data analysis, image processing, and other research
fields. Among sparse representations, the cosparse analysis model has recently
gained increasing interest. Many signals exhibit a multidimensional structure,
e.g. images or three-dimensional MRI scans. Most data analysis and learning
algorithms use vectorized signals and thereby do not account for this
underlying structure. The drawback of not taking the inherent structure into
account is a dramatic increase in computational cost. We propose an algorithm
for learning a cosparse Analysis Operator that adheres to the preexisting
structure of the data, and thus allows for a very efficient implementation.
This is achieved by enforcing a separable structure on the learned operator.
Our learning algorithm is able to deal with multidimensional data of arbitrary
order. We evaluate our method on volumetric data at the example of
three-dimensional MRI scans.Comment: 5 pages, 3 figures, accepted at EUSIPCO 201
Learning Co-Sparse Analysis Operators with Separable Structures
In the co-sparse analysis model a set of filters is applied to a signal out
of the signal class of interest yielding sparse filter responses. As such, it
may serve as a prior in inverse problems, or for structural analysis of signals
that are known to belong to the signal class. The more the model is adapted to
the class, the more reliable it is for these purposes. The task of learning
such operators for a given class is therefore a crucial problem. In many
applications, it is also required that the filter responses are obtained in a
timely manner, which can be achieved by filters with a separable structure. Not
only can operators of this sort be efficiently used for computing the filter
responses, but they also have the advantage that less training samples are
required to obtain a reliable estimate of the operator. The first contribution
of this work is to give theoretical evidence for this claim by providing an
upper bound for the sample complexity of the learning process. The second is a
stochastic gradient descent (SGD) method designed to learn an analysis operator
with separable structures, which includes a novel and efficient step size
selection rule. Numerical experiments are provided that link the sample
complexity to the convergence speed of the SGD algorithm.Comment: 11 pages double column, 4 figures, 3 table
An algorithm for sums of squares of real polynomials
AbstractWe present an algorithm to determine if a real polynomial is a sum of squares (of polynomials), and to find an explicit representation if it is a sum of squares. This algorithm uses the fact that a sum of squares representation of a real polynomial corresponds to a real, symmetric, positive semi-definite matrix whose entries satisfy certain linear equations
Tabakentwöhnung in der erwachsenen Allgemeinbevölkerung : Evaluation eines internetbasierten Ausstiegsprogramms für Raucher.
Wörmann M. Tabakentwöhnung in der erwachsenen Allgemeinbevölkerung : Evaluation eines internetbasierten Ausstiegsprogramms für Raucher. Bielefeld: Universitätsbibliothek Bielefeld; 2014
Aspects of nonequilibrium in Leptogenesis
Wörmann M. Aspects of nonequilibrium in Leptogenesis. Bielefeld: Universität Bielefeld; 2016
Non-relativistic leptogenesis
Bödeker D, Wörmann M. Non-relativistic leptogenesis. Journal of Cosmology and Astroparticle Physics. 2014;2014(02):016.In many phenomenologically interesting models of thermal leptogenesis the heavy neutrinos are non-relativistic when they decay and produce the baryon asymmetry of the Universe. We propose a non-relativistic approximation for the corresponding rate equations in the non-resonant case, and a systematic way for computing relativistic corrections. We determine the leading order coefficients in these equations, and the first relativistic corrections. The non-relativistic approximation works remarkably well. It appears to be consistent with results obtained using a Boltzmann equation taking into account the momentum distribution of the heavy neutrinos, while being much simpler. We also compute radiative corrections to some of the coefficients in the rate equations. Their effect is of order 1% in the regime favored by neutrino oscillation data. We obtain the correct leading order lepton number washout rate in this regime, which leads to large (~ 20%) effects compared to previous computations
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