205 research outputs found
Bioinformatic evaluation of L-arginine catabolic pathways in 24 cyanobacteria and transcriptional analysis of genes encoding enzymes of L-arginine catabolism in the cyanobacterium Synechocystis sp. PCC 6803
No full textSchriek S, Rückert C, Staiger D, Pistorius EK, Michel K-P. Bioinformatic evaluation of L-arginine catabolic pathways in 24 cyanobacteria and transcriptional analysis of genes encoding enzymes of L-arginine catabolism in the cyanobacterium Synechocystis sp. PCC 6803. BMC Genomics. 2007;8(1): 437.BACKGROUND:So far very limited knowledge exists on L-arginine catabolism in cyanobacteria, although six major L-arginine-degrading pathways have been described for prokaryotes. Thus, we have performed a bioinformatic analysis of possible L-arginine-degrading pathways in cyanobacteria. Further, we chose Synechocystis sp. PCC 6803 for a more detailed bioinformatic analysis and for validation of the bioinformatic predictions on L-arginine catabolism with a transcript analysis.RESULTS:We have evaluated 24 cyanobacterial genomes of freshwater or marine strains for the presence of putative L-arginine-degrading enzymes. We identified an L-arginine decarboxylase pathway in all 24 strains. In addition, cyanobacteria have one or two further pathways representing either an arginase pathway or L-arginine deiminase pathway or an L-arginine oxidase/dehydrogenase pathway. An L-arginine amidinotransferase pathway as a major L-arginine-degrading pathway is not likely but can not be entirely excluded. A rather unusual finding was that the cyanobacterial L-arginine deiminases are substantially larger than the enzymes in non-photosynthetic bacteria and that they are membrane-bound. A more detailed bioinformatic analysis of Synechocystis sp. PCC 6803 revealed that three different L-arginine-degrading pathways may in principle be functional in this cyanobacterium. These are (i) an L-arginine decarboxylase pathway, (ii) an L-arginine deiminase pathway, and (iii) an L-arginine oxidase/dehydrogenase pathway. A transcript analysis of cells grown either with nitrate or L-arginine as sole N-source and with an illumination of 50 mumol photons m-2 s-1 showed that the transcripts for the first enzyme(s) of all three pathways were present, but that the transcript levels for the L-arginine deiminase and the L-arginine oxidase/dehydrogenase were substantially higher than that of the three isoenzymes of L-arginine decarboxylase.CONCLUSION:The evaluation of 24 cyanobacterial genomes revealed that five different L-arginine-degrading pathways are present in the investigated cyanobacterial species. In Synechocystis sp. PCC 6803 an L-arginine deiminase pathway and an L-arginine oxidase/dehydrogenase pathway represent the major pathways, while the L-arginine decarboxylase pathway most likely only functions in polyamine biosynthesis. The transcripts encoding the enzymes of the two major pathways were constitutively expressed with the exception of the transcript for the carbamate kinase, which was substantially up-regulated in cells grown with L-arginine
Some remarks on first passage of Lévy processes, the American put and pasting principles
No full textThe purpose of this article is to provide, with the help of a fluctuation identity, a generic link between a number of known identities for the first passage time and overshoot above/below a fixed level of a Lévy process and the solution of Gerber and Shiu [Astin Bull. 24 (1994) 195–220], Boyarchenko and Levendorskii [Working paper series EERS 98/02 (1998), Unpublished manuscript (1999), SIAM J. Control Optim. 40 (2002) 1663–1696], Chan [Original unpublished manuscript (2000)], Avram, Chan and Usabel [Stochastic Process. Appl. 100 (2002) 75–107], Mordecki [Finance Stoch. 6 (2002) 473–493], Asmussen, Avram and Pistorius [Stochastic Process. Appl. 109 (2004) 79–111] and Chesney and Jeanblanc [Appl. Math. Fin. 11 (2004) 207–225] to the American perpetual put optimal stopping problem. Furthermore, we make folklore precise and give necessary and sufficient conditions for smooth pasting to occur in the considered problem
On the optimal dividend problem for a spectrally negative L\'{e}vy process
No full textIn this paper we consider the optimal dividend problem for an insurance company whose risk process evolves as a spectrally negative L\'{e}vy process in the absence of dividend payments. The classical dividend problem for an insurance company consists in finding a dividend payment policy that maximizes the total expected discounted dividends. Related is the problem where we impose the restriction that ruin be prevented: the beneficiaries of the dividends must then keep the insurance company solvent by bail-out loans. Drawing on the fluctuation theory of spectrally negative L\'{e}vy processes we give an explicit analytical description of the optimal strategy in the set of barrier strategies and the corresponding value function, for either of the problems. Subsequently we investigate when the dividend policy that is optimal among all admissible ones takes the form of a barrier strategy.
Reproductive phenology is a repeatable, heritable trait linked to the timing of other life history events in a migratory marine predator
No full textPopulation-level shifts in reproductive phenology in response to environmental change are common, but whether individual-level responses are modified by demographic and genetic factors remains less well understood. We used mixed models to quantify how reproductive timing varied across 1,755 female southern elephant seals (Mirounga leonina) breeding at Marion Island in the Southern Ocean (1989–2019) and to identify the factors that correlate with phenological shifts within- and between individuals. We found strong support for covariation in the timing of breeding arrival dates and the timing of the preceding moult. Breeding arrival dates were more repeatable at the individual-level, as compared to the population-level, even after accounting for individual traits (wean date as a pup, age and breeding experience) associated with phenological variability. Mother-daughter similarities in breeding phenology were also evident, indicating that additive genetic effects may contribute to between-individual variation in breeding phenology. Over 30 years, elephant seal phenology did not change towards earlier or later dates, and we found no correlation between annual fluctuations in phenology and indices of environmental variation. Our results show how maternal genetic (or non-genetic) effects, individual traits and linkages between cyclical life-history events can drive within- and between-individual variation in reproductive phenology.Data were processed in R 4.1.3 and later versions (R Core Team 2022). Anyone interested in using this long-term data is encouraged to collaborate with the authors, as their understanding of the data collection methods and availability of other meta-data is likely to enhance future analysis and interpretation.Funding provided by: National Research FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100001321Award Number:From 1983 to 2019, nearly all elephant seal pups born at Marion Island were uniquely marked with two hind-flipper tags (n = 8,985 female pups) (Bester et al. 2011, Pistorius et al. 2011). The pups were marked at the first encounter after weaning, after their mothers had abandoned them. However, since 2006, we also marked some pre-weaned pups with ancillary tags (de Bruyn et al. 2008), thereby collecting pedigree information on 2,115 mother-daughter pairs. Data were processed in R 4.1.3 and later versions (R Core Team 2022) (see R script).
Bester MN, de Bruyn PJN, Oosthuizen WC, Tosh CA, McIntyre T, Reisinger, RR et al. 2011 The Marine Mammal Programme at the Prince Edward Islands: 38 years of research. Afr. J. Mar. Sci. 33, 511-521.
De Bruyn PJN, Tosh CA, Oosthuizen WC, Phalanndwa MV, Bester MN. 2008 Temporary marking of unweaned southern elephant seal (Mirounga leonina L.) pups. S. Afr. J. Wildl. Res. 38, 133-137.
Pistorius PA, de Bruyn PJN, Bester MN. 2011 Population dynamics of southern elephant seals: a synthesis of three decades of demographic research at Marion Island. Afr. J. Mar. Sci. 33, 523-534.
R Core Team. 2022 R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. URL https://www.R-project.org/
On the optimal dividend problem for a spectrally negative L\'{e}vy process
No full textPublished at http://dx.doi.org/10.1214/105051606000000709 in the Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute of Mathematical Statistics (http://www.imstat.org)In this paper we consider the optimal dividend problem for an insurance company whose risk process evolves as a spectrally negative L\'{e}vy process in the absence of dividend payments. The classical dividend problem for an insurance company consists in finding a dividend payment policy that maximizes the total expected discounted dividends. Related is the problem where we impose the restriction that ruin be prevented: the beneficiaries of the dividends must then keep the insurance company solvent by bail-out loans. Drawing on the fluctuation theory of spectrally negative L\'{e}vy processes we give an explicit analytical description of the optimal strategy in the set of barrier strategies and the corresponding value function, for either of the problems. Subsequently we investigate when the dividend policy that is optimal among all admissible ones takes the form of a barrier strategy
An Insight into the acceptable use and assessment of lower-limb running prostheses in disability sport.
No full textSports technology can be any product or system used to facilitate, train or influence an athlete’s performance. The role of prostheses used for disability sport was initially to help facilitate exercise and then ultimately, competition. In able-bodied sport, controversy has occasionally been caused through the adoption or introduction of sports technology. However, scant attention has been paid to sport with a disability with respect to such concerns. This research project provides a novel contribution to knowledge by investigating the use of lower-limb running prostheses in competition by trans-tibial amputees. A novel study using a mixed method approach has investigated the nature, use and assessment of lower-limb running prostheses. It has proposed that the unchecked introduction of such technology has affected the sport negatively. From this, the study conducted a stakeholder assessment of the sport and provided a proposed series of guidelines for lower-limb prostheses technology inclusion. Finally, the recommendation was made that a proactive approach to such technologies’ inclusion in the future should be implemented. These guidelines were further developed by assessing symmetrical and nonsymmetrical lower-limb function and proposed that single and double lowerlimb amputees should be separated in competition in the future. To this end, it was proposed that lower-limb symmetry, stiffness and energy return were important means of monitoring prosthesis performance. Ultimately, a dynamic technique which assesses these qualities was proposed as an assessment strategy for further development in the future
On optimality of the barrier strategy in de Finetti's dividend problem for spectrally negative Lévy processes
No full textWe consider the classical optimal dividend control problem which was proposed by de Finetti [Trans. XVth Internat. Congress Actuaries 2 (1957) 433-443], Recently Avram, Palmowski and Pistorius [Ann. Appl. Probab. 17 (2007) 156-180] studied the case when the risk process is modeled by a general spectrally negative Lévy process. We draw upon their results and give sufficient conditions under which the optimal strategy is of barrier type, thereby helping to explain the fact that this particular strategy is not optimal in general. As a consequence, we are able to extend considerably the class of processes for which the barrier strategy proves to be optimal. © Institute of Mathematical Statistics, 2008
On the drawdown of completely asymmetric Levy processes
No full textThe {\em drawdown} process of a completely asymmetric L\'{e}vy process is equal to reflected at its running supremum : . In this paper we explicitly express in terms of the scale function and the L\'{e}vy measure of the law of the sextuple of the first-passage time of over the level , the time of the last supremum of prior to , the infimum \unl X_{\tau_a} and supremum \ovl X_{\tau_a} of at and the undershoot and overshoot of at . As application we obtain explicit expressions for the laws of a number of functionals of drawdowns and rallies in a completely asymmetric exponential L\'{e}vy model.
On future drawdowns of Levy processes
No full textFor a given Levy process
X
= (
X
t
)
t
2
R
+
and for xed
s
2
R
+
[f1g
and
t
2
R
+
we analyse the
future drawdown extremes
that are de ned as follows:
The path-functionals
D
t;s
and
D
t;s
are of interest in various areas of application, including nancial mathematics
and queueing theory. In the case that
X
has a strictly positive mean, we nd the exact asymptotic decay as
x
! 1
of the tail probabilities
P
(
D
t
< x
) and
P
(
D
t
< x
) of
D
t
= lim
s
!1
D
t;s
and
D
t
= lim
s
!1
D
t;s
both when the jumps satisfy the Cram er assumption and in a heavy-tailed case. Furthermore, in the case that
the jumps of the L evy process
X
are of single sign and
X
is not subordinator, we identify the one-dimensional
distributions in terms of the scale function of
X
. By way of example, we derive explicit results for the Black-
Scholes-Samuelson model
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