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101.
When deprived of combined nitrogen, aerobically-grown filaments ofAnabaena sp. strain PCC7120 differentiate specialized cells called the heterocysts. The differentiation process is an elaborate and
well orchestrated programme involving sensing of environmental and developmental signals, commitment of cells to development,
gene rearrangements, intricate DNA-protein interactions, and differential expression of several genes. It culminates in a
physiological division of labour between heterocysts, which become the sole sites of aerobic nitrogen fixation, and vegetative
cells, that provide photosynthate to the heterocysts in return for nitrogen supplies. We propose a model, to describe the
chronology of the important events and to explain how cell type-specific differential gene expression is facilitated by DNA-protein
interactions leading to the development of heterocysts and constitution of nitrogen-fixing apparatus inAnabaena. 相似文献
102.
V. Kumar J. L. de la Fuente A. L. Leitão P. Liras J. F. Martin 《Applied microbiology and biotechnology》1996,45(5):621-628
The bla gene of the cephamycin cluster of Nocardia lactamdurans has been subcloned in the shuttle plasmids pULVK2 and pULVK2A and amplified in N. lactamdurans LC411. The transformants showed two- to threefold higher β-lactamase activity. Formation of β-lactamase preceded the onset
of cephamycin biosynthesis. The β-lactamase of N. lactamdurans inactivated penicillins and, to a lesser extent, cephalosporin C but did not hydrolyse cephamycin C. This β-lactamase was
highly sensitive to clavulanic acid (50% inhibition was observed at 0.48 μg/ml clavulanic acid). The N. lactamdurans bla gene was disrupted in vivo by inertion of the kanamycin-resistance gene. Three bla-disrupted mutants, BD4, BD8 and BD12, were selected that lacked β-lactamase activity. Overexpresion of the bla gene resulted in N. lactamdurans transformants that were resistant to penicillin whereas mutants in which the bla gene was disrupted were supersensitive to this antibiotic. The three N. lactamdurans mutants with the bla gene disrupted showed a significant increase of cephamycin biosynthesis in solid medium, whereas transformants with the amplified
bla gene produced reduced levels of cephamycin. The cephamycin-overproducing Merck strain N. lactamdurans MA4213 showed no detectable levels of β-lactamase activity. The β-lactamase plays a negative role in cephamycin biosynthesis
in solid medium, but not in liquid medium.
Received: 26 July 1995/Received revision: 18 December 1995/Accepted: 8 January 1996 相似文献
103.
Different submodels within complex model packages on N regimes-for plant N-uptake, net N-mineralization, nitrate leaching and microbial N immobilization-are critically reviewed mainly with regard to their prediction ability on the basis of three comparative papers. Only for some of the processes adequate statistical evaluation of the models was possible. Compared to the other statistically evaluable process, nitrate leaching, modeling of plant N-uptake yields the better results. Most models for mineralization use arbitrary approaches rather than empirical ones. Although only approximate estimates of N mineralisation were at hand, the models generally behave expectedly poor. Only one model-DAISY-out of 16 involved in the comparison uses an explicit microbial biomass sub-model including microbial growth, decline and maintenance terms. So DAISY is the only model coupling C and N cycles. But what is true for an individual model describing the C and N transformation of a lab incubation experiment seems to be valid for most of the complex simulation work on the C and N regimes: this model was said to be overparameterized with respect to the available data. 相似文献
104.
A 43 kDa DNA binding protein from the pea chloroplast interacts with and stimulates the cognate DNA polymerase. 下载免费PDF全文
W Chen A Gaikwad S K Mukherjee N R Choudhary D Kumar K K Tewari 《Nucleic acids research》1996,24(20):3953-3961
A DNA binding protein with DNA polymerase 'accessory activity' has been identified and purified to apparent homogeneity from pea chloroplasts. This protein consists of a single subunit of 43 kDa and binds to DNA regardless of its base sequence and topology. It increases cognate DNA polymerase-primase activity in a dose dependent manner. Using solid phase protein-protein interaction trapping and co-immunoprecipitation techniques, the purified protein was found to associate with the chloroplast DNA polymerase. The chloroplast DNA polymerase also binds directly to the radioiodinated 43 kDa protein. The specific interaction between 43 kDa protein and chloroplast DNA polymerase results in the synthesis of longer DNA chains. The 43 kDa protein, present abundantly in the pea chloroplast, appears to increase processivity of the chloroplast DNA polymerase and may play an important role in the replication of pea chloroplast DNA. 相似文献
105.
Action potential conduction between a ventricular cell model and an isolated ventricular cell. 总被引:4,自引:1,他引:3
R Wilders R Kumar R W Joyner H J Jongsma E E Verheijck D Golod A C van Ginneken W N Goolsby 《Biophysical journal》1996,70(1):281-295
We used the Luo and Rudy (LR) mathematical model of the guinea pig ventricular cell coupled to experimentally recorded guinea pig ventricular cells to investigate the effects of geometrical asymmetry on action potential propagation. The overall correspondence of the LR cell model with the recorded real cell action potentials was quite good, and the strength-duration curves for the real cells and for the LR model cell were in general correspondence. The experimental protocol allowed us to modify the effective size of either the simulation model or the real cell. 1) When we normalized real cell size to LR model cell size, required conductance for propagation between model cell and real cell was greater than that found for conduction between two LR model cells (5.4 nS), with a greater disparity when we stimulated the LR model cell (8.3 +/- 0.6 nS) than when we stimulated the real cell (7.0 +/- 0.2 nS). 2) Electrical loading of the action potential waveform was greater for real cell than for LR model cell even when real cell size was normalized to be equal to that of LR model cell. 3) When the size of the follower cell was doubled, required conductance for propagation was dramatically increased; but this increase was greatest for conduction from real cell to LR model cell, less for conduction from LR model cell to real cell, and least for conduction from LR model cell to LR model cell. The introduction of this "model clamp" technique allows testing of proposed membrane models of cardiac cells in terms of their source-sink behavior under conditions of extreme coupling by examining the symmetry of conduction of a cell pair composed of a model cell and a real cardiac cell. We have focused our experimental work with this technique on situations of extreme uncoupling that can lead to conduction block. In addition, the analysis of the geometrical factors that determine success or failure of conduction is important in the understanding of the process of discontinuous conduction, which occurs in myocardial infarction. 相似文献
106.
Patterns of Nucleotide Substitution in Mitochondrial Protein Coding Genes of Vertebrates 总被引:7,自引:0,他引:7 下载免费PDF全文
S. Kumar 《Genetics》1996,143(1):537-548
Maximum likelihood methods were used to study the differences in substitution rates among the four nucleotides and among different nucleotide sites in mitochondrial protein-coding genes of vertebrates. In the 1st+2nd codon position data, the frequency of nucleotide G is negatively correlated with evolutionary rates of genes, substitution rates vary substantially among sites, and the transition/transversion rate bias (R) is two to five times larger than that expected at random. Generally, largest transition biases and greatest differences in substitution rates among sites are found in the highly conserved genes. The 3rd positions in placental mammal genes exhibit strong nucleotide composition biases and the transitional rates exceed transversional rates by one to two orders of magnitude. Tamura-Nei and Hasegawa-Kishino-Yano models with gamma distributed variable rates among sites (gamma parameter, α) adequately describe the nucleotide substitution process in 1st+2nd position data. In these data, ignoring differences in substitution rates among sites leads to largest biases while estimating substitution rates. Kimura's two-parameter model with variable-rates among sites performs satisfactorily in likelihood estimation of R, α, and overall amount of evolution for 1st+2nd position data. It can also be used to estimate pairwise distances with appropriate values of α for a majority of genes. 相似文献
107.
A stepwise algorithm for finding minimum evolution trees 总被引:7,自引:6,他引:1
A stepwise algorithm for reconstructing minimum evolution (ME) trees from
evolutionary distance data is proposed. In each step, a taxon that
potentially has a neighbor (another taxon connected to it with a single
interior node) is first chosen and then its true neighbor searched
iteratively. For m taxa, at most (m-1)!/2 trees are examined and the tree
with the minimum sum of branch lengths (S) is chosen as the final tree.
This algorithm provides simple strategies for restricting the tree space
searched and allows us to implement efficient ways of dynamically computing
the ordinary least squares estimates of S for the topologies examined.
Using computer simulation, we found that the efficiency of the ME method in
recovering the correct tree is similar to that of the neighbor-joining
method (Saitou and Nei 1987). A more exhaustive search is unlikely to
improve the efficiency of the ME method in finding the correct tree because
the correct tree is almost always included in the tree space searched with
this stepwise algorithm. The new algorithm finds trees for which S values
may not be significantly different from that of the ME tree if the correct
tree contains very small interior branches or if the pairwise distance
estimates have large sampling errors. These topologies form a set of
plausible alternatives to the ME tree and can be compared with each other
using statistical tests based on the minimum evolution principle. The new
algorithm makes it possible to use the ME method for large data sets.
相似文献
108.
Approximate methods for estimating the pattern of nucleotide substitution and the variation of substitution rates among sites 总被引:13,自引:6,他引:7
We propose two approximate methods (one based on parsimony and one on
pairwise sequence comparison) for estimating the pattern of nucleotide
substitution and a parsimony-based method for estimating the gamma
parameter for variable substitution rates among sites. The matrix of
substitution rates that represents the substitution pattern can be
recovered through its relationship with the observable matrix of site
pattern frequences in pairwise sequence comparisons. In the parsimony
approach, the ancestral sequences reconstructed by the parsimony algorithm
were used, and the two sequences compared are those at the ends of a branch
in the phylogenetic tree. The method for estimating the gamma parameter was
based on a reinterpretation of the numbers of changes at sites inferred by
parsimony. Three data sets were analyzed to examine the utility of the
approximate methods compared with the more reliable likelihood methods. The
new methods for estimating the substitution pattern were found to produce
estimates quite similar to those obtained from the likelihood analyses. The
new method for estimating the gamma parameter was effective in reducing the
bias in conventional parsimony estimates, although it also overestimated
the parameter. The approximate methods are computationally very fast and
appear useful for analyzing large data sets, for which use of the
likelihood method requires excessive computation.
相似文献
109.
Jawahar Kalra Subrahmanyam V. Mantha Praveen Kumar Kailash Prasad 《Molecular and cellular biochemistry》1994,136(2):125-129
Lipid peroxidation of membranes by oxygen free radicals has been implicated in various disease states. Different antioxidants and iron chelators have been used to reduce lipid peroxidation. Lazaroids have been used for the acute treatment of central nervous system disorders such as trauma and ischemia wherein lipid peroxidative processes take place.In this study we evaluated the effect of lazaroids (U-785 18F and U-74389F) on the release of acid phosphatase activity and formation of malondialdehyde (MDA) in rat liver lyosomes subjected to exogenously generated oxygen free radicals. There was a significant increase in the acid phosphatase release and MDA formation in the presence of oxygen free radicals. This was prevented by both the lazaroids. In a separate study the effect of lazaroid U-74389F was seen on the zymosan-stimulated polymorphonuclear (PMN) leukocyte-derived chemiluminescence. The PMN leukocyte chemiluminescent activity was attenuated by the lazaroid in a dose-dependent manner. These studies suggest that lazaroids may inhibit lipid peroxidation and stabilize the membrane. 相似文献
110.