1,317 research outputs found
Determination of mean surface position and sea state from the radar return of a short-pulse satellite altimeter
Using the specular point theory of scatter from a very rough surface, the average backscatter cross section per unit area per radar cell width is derived for a cell located at a given height above the mean sea surface. This result is then applied to predict the average radar cross section observed by a short-pulse altimeter as a function of time for two modes of operation: pulse-limited and beam-limited configurations. For a pulse-limited satellite altimeter, a family of curves is calculated showing the distortion of the leading edge of the receiver output signal as a function of sea state (i.e., wind speed). A signal processing scheme is discussed that permits an accurate determination of the mean surface position--even in high seas--and, as a by-product, the estimation of the significant seawave height (or wind speed above the surface). Comparison of these analytical results with experimental data for both pulse-limited and beam-limited operation lends credence to the model. Such a model should aid in the design of short-pulse altimeters for accurate determination of the geoid over the oceans, as well as for the use of such altimeters for orbital sea-state monitoring
Determination of RMS height of a rough surface using radar waves
Root mean square height of rough surface determined by measuring correlation between two scattered radar waves at different frequencies as function of frequency separatio
Scattering from surfaces with different roughness scales, analysis and interpretation
Statistical analysis and physical interpretation of scattering from surfaces with different roughness scale
SYSTEMS ANALYSIS APPROACH TO SELECTION OF FARM EQUIPMENT
Farm Management,
Reduced mutation rate and increased transformability of transposon-free Acinetobacter baylyi ADP1-ISx
ABSTRACT
The genomes of most bacteria contain mobile DNA elements that can contribute to undesirable genetic instability in engineered cells. In particular, transposable insertion sequence (IS) elements can rapidly inactivate genes that are important for a designed function. We deleted all six copies of IS
1236
from the genome of the naturally transformable bacterium
Acinetobacter baylyi
ADP1. The natural competence of ADP1 made it possible to rapidly repair deleterious point mutations that arose during strain construction. In the resulting ADP1-ISx strain, the rates of mutations inactivating a reporter gene were reduced by 7- to 21-fold. This reduction was higher than expected from the incidence of new IS
1236
insertions found during a 300-day mutation accumulation experiment with wild-type ADP1 that was used to estimate spontaneous mutation rates in the strain. The extra improvement appears to be due in part to eliminating large deletions caused by IS
1236
activity, as the point mutation rate was unchanged in ADP1-ISx. Deletion of an error-prone polymerase (
dinP
) and a DNA damage response regulator (
umuD
Ab
[the
umuD
gene of
A. baylyi
]) from the ADP1-ISx genome did not further reduce mutation rates. Surprisingly, ADP1-ISx exhibited increased transformability. This improvement may be due to less autolysis and aggregation of the engineered cells than of the wild type. Thus, deleting IS elements from the ADP1 genome led to a greater than expected increase in evolutionary reliability and unexpectedly enhanced other key strain properties, as has been observed for other clean-genome bacterial strains. ADP1-ISx is an improved chassis for metabolic engineering and other applications.
IMPORTANCE
Acinetobacter baylyi
ADP1 has been proposed as a next-generation bacterial host for synthetic biology and genome engineering due to its ability to efficiently take up DNA from its environment during normal growth. We deleted transposable elements that are capable of copying themselves, inserting into other genes, and thereby inactivating them from the ADP1 genome. The resulting āclean-genomeā ADP1-ISx strain exhibited larger reductions in the rates of inactivating mutations than expected from spontaneous mutation rates measured via whole-genome sequencing of lineages evolved under relaxed selection. Surprisingly, we also found that IS element activity reduces transformability and is a major cause of cell aggregation and death in wild-type ADP1 grown under normal laboratory conditions. More generally, our results demonstrate that domesticating a bacterial genome by removing mobile DNA elements that have accumulated during evolution in the wild can have unanticipated benefits.
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Physical Model of the Immune Response of Bacteria Against Bacteriophage Through the Adaptive CRISPR-Cas Immune System
Bacteria and archaea have evolved an adaptive, heritable immune system that
recognizes and protects against viruses or plasmids. This system, known as the
CRISPR-Cas system, allows the host to recognize and incorporate short foreign
DNA or RNA sequences, called `spacers' into its CRISPR system. Spacers in the
CRISPR system provide a record of the history of bacteria and phage
coevolution. We use a physical model to study the dynamics of this coevolution
as it evolves stochastically over time. We focus on the impact of mutation and
recombination on bacteria and phage evolution and evasion. We discuss the
effect of different spacer deletion mechanisms on the coevolutionary dynamics.
We make predictions about bacteria and phage population growth, spacer
diversity within the CRISPR locus, and spacer protection against the phage
population.Comment: 37 pages, 13 figure
Theoretical curves of backscattering cross sections of rough surfaces for several polarization states using two statistical models
Theoretical curves of backscattering cross sections of rough surfaces for polarization states using statistical model
A more exact theory of backscattering from statistically rough surfaces
Physical optics and other approximations examined for improving and extending theory of electromagnetic backscattering from rough surface
Two experiments yielding lunar surface information employing polarized radar waves
Polarized radar waves used for determining local statistical electromagnetic backscattering properties of lunar or other planetary surfac
Summary of concepts and transformations commonly used in the matrix description of polarized waves
Concepts and transformations commonly used in matrix description of polarized wave
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