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The phylogenetic position of the archaebacteria and the place of eukaryotes
in the history of life remain a question of debate. Recent studies based on
some protein-sequence data have obtained unusual phylogenies for these
organisms. We therefore collected the protein sequences that were available
with representatives from each of the major forms of life: the
gram-negative bacteria, gram-positive bacteria, archaebacteria, and
eukaryotes. Monophyletic, unrooted phylogenies based on these sequence data
show that seven of 24 proteins yield a significant
gram-positive-archaebacteria clade/gram-negative- eukaryotic clade. The
phylogenies for these seven proteins cannot be explained by the traditional
three-way split of the eukaryotes, archaebacteria, and eubacteria. Nine of
the 24 proteins yield the traditional gram-positive-gram-negative
clade/archaebacteria-eukaryotic clade. The remaining eight proteins give
phylogenies that cannot be statistically distinguished. These results
support the hypothesis of a chimeric origin for the eukaryotic cell nucleus
formed from the fusion of an archaebacteria and a gram-negative bacteria.
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Calcium control of waveform in isolated flagellar axonemes of chlamydomonas 总被引:29,自引:24,他引:5
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The effect of Ca(++) on the waveform of reactivated, isolated axonemes of chlamydomonas flagella was investigated. Flagella were detached and isolated by the dibucaine procedure and demembranated by treatment with the detergent Nonidet; the resulting axomenes lack the flagellar membrane and basal bodies. In Ca(++)-buffered reactivation solutions containing 10(-6) M or less free Ca(++), the axonemes beat with a highly asymmetrical, predominantly planar waveform that closely resembled that of in situ flagella of forward swimming cells. In solutions containing 10(-4) M Ca(++), the axonemes beat with a symmetrical waveform that was very similar to that of in situ flagella during backward swimming. In 10(-5) M Ca(++), the axonemes were predominantly quiescent, a state that appears to be closely associated with changes in axomenal waveform or direction of beat in many organisms. Experiments in which the concentrations of free Ca(++), not CaATP(--) complex were independently varied suggested that free Ca(++), not CaATP(--), was responsible for the observed changes. Analysis of the flagellar ATPases associated with the isolated axonemes and the nonidet- soluble membrane-matrix fraction obtained during preparation of the axonemes showed that the axonemes lacked the 3.0S Ca(++)-activated ATPase, almost all of which was recovered in the membrane-matrix fraction. These results indicate that free Ca(++) binds directly to an axonemal component to alter flagellar waveform, and that neither the 3.0S CaATPase nor the basal bodies are directly involved in this change. 相似文献
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The mosaic nature of the eukaryotic nucleus 总被引:6,自引:1,他引:5
The phylogenies for each of the protein-coding genes from the Methanococcus
jannaschii genome were surveyed to determine the history of the major
groups of life. For each gene, homologous sequences from other archaea,
eucarya, and Gram-positive and Gram-negative bacteria were collected and
aligned, and a phylogeny was reconstructed with a maximum-likelihood
algorithm. The majority of significant phylogenies favor the eucarya and
the archaca as sister groups. A smaller, but still substantial, portion of
these significant phylogenies favor an eucarya/Gram-negative clade. These
results indicate that support for the early history of life is not
unequivocal. A chimeric origin of eukaryotes or an ancient, massive
horizontal transfer of genes from Gram-negative bacteria to eucarya can
explain many of the observed phylogenies.
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Several studies have indicated that olfactory responses are impeded by
amiloride. Therefore, it was of interest to see whether, and if so which,
olfactory epithelial cellular compartments have amiloride- sensitive
structures. Using ultrastructural methods that involved rapid freezing,
freeze-substitution and low temperature embedding of olfactory epithelia,
this study shows that, in the rat, this tissue is immunoreactive to
antibodies against amiloride sensitive Na(+)- channels. However, microvilli
of olfactory supporting cells, as opposed to receptor cilia, contained most
of the immunoreactive sites. Apices from which the microvilli sprout and
receptor cell dendritic knobs had much less if any of the
amiloride-antibody binding sites. Using a direct ligand-binding
cytochemical method, this study also confirms earlier ones that showed that
olfactory receptor cell cilia have Na+, K(+)-ATPase. It is proposed that
supporting cell microvilli and the receptor cilia themselves have
mechanisms, different but likely complementary, that participate in
regulating the salt concentration around the receptor cell cilia. In this
way, both structures help to provide the ambient mucous environment for
receptor cells to function properly. This regulation of the salt
concentration of an ambient fluid environment is a function that the
olfactory epithelium shares with cells of transporting epithelia, such as
those of kidney.
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Sharma Ayushi Malhotra Bhanu Kharkwal Harsha Kulkarni Giriraj T. Kaushik Nutan 《Phytochemistry Reviews》2020,19(3):691-720
Phytochemistry Reviews - Endophytes are a diverse group of microorganisms present in plant tissues which include fungi and bacteria and have been utilized as therapeutic agents. They are known to... 相似文献
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