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161.
Molecular coevolution among cryptically simple expansion segments of eukaryotic 26S/28S rRNAs 总被引:16,自引:4,他引:12
The set of "expansion segments" of any eukaryotic 26S/28S ribosomal RNA
(rRNA) gene is responsible for the bulk of the difference in length between
the prokaryotic 23S rRNA gene and the eukaryotic 26S/28S rRNA gene. The
expansion segments are also responsible for interspecific fluctuations in
length during eukaryotic evolution. They show a consistent bias in base
composition in any species; for example, they are AT rich in Drosophila
melanogaster and GC rich in vertebrate species. Dot-matrix comparisons of
sets of expansion segments reveal high similarities between members of a
set within any 28S rRNA gene of a species, in contrast to the little or
spurious similarity that exists between sets of expansion segments from
distantly related species. Similarities among members of a set of expansion
segments within any 28S rRNA gene cannot be accounted for by their
base-compositional bias alone. In contrast, no significant similarity
exists within a set of "core" segments (regions between expansion segments)
of any 28S rRNA gene, although core segments are conserved between species.
The set of expansion segments of a 26S/28S gene is coevolving as a unit in
each species, at the same time as the family of 28S rRNA genes, as a whole,
is undergoing continual homogenization, making all sets of expansion
segments from all ribosomal DNA (rDNA) arrays in a species similar in
sequence. Analysis of DNA simplicity of 26S/28S rRNA genes shows a direct
correlation between significantly high relative simplicity factors (RSFs)
and sequence similarity among a set of expansion segments. A similar
correlation exists between RSF values, overall rDNA lengths, and the
lengths of individual expansion segments. Such correlations suggest that
most length fluctuations reflect the gain and loss of simple sequence
motifs by slippage-like mechanisms. We discuss the molecular coevolution of
expansion segments, which takes place against a background of slippage-like
and unequal crossing-over mechanisms of turnover that are responsible for
the accumulation of interspecific differences in rDNA sequences.
相似文献
162.
Phosphoribosyl anthranilate isomerase from Thermotoga maritima is an extremely stable and active homodimer. 总被引:1,自引:1,他引:0
R. Sterner G. R. Kleemann H. Szadkowski A. Lustig M. Hennig K. Kirschner 《Protein science : a publication of the Protein Society》1996,5(10):2000-2008
The metabolism of hyperthermophilic microorganisms can function properly at temperatures close to 100 degrees C. It follows that they are equipped with both thermostable enzymes and mechanisms that handle labile metabolites. We wanted to understand how stable and active phosphoribosyl anthranilate isomerase (tPRAI) from the hyperthermophile Thermotoga maritima is at its optimum growth temperature of 80 degrees C, and how its thermolabile substrate, N-(5'-phosphoribosyl)-anthranilate (PRA), is protected from rapid decomposition. To this end, the trpF gene of T. maritima was expressed heterologously in Escherichia coli and tPRAI was purified. In contrast to most PRAIs from mesophiles, which are monomers with the eightfold beta alpha (or TIM) barrel fold, tPRAI is a homodimer. It is strongly resistant toward inactivation by temperatures up to 95 degrees C, by acidification to pH 3.2, and by proteases in the presence and absence of detergents. tPRAI is about 35-fold more active at its physiologic temperature than is the enzyme from E. coli (ePRAI) at 37 degrees C. This high catalytic efficiency of tPRAI is likely to complete successfully with the rapid spontaneous hydrolysis of PRA at 80 degrees C. Thus, with respect to both stability and function, tPRAI appears well adapted to the extreme habitat of T. maritima. Single crystals of tPRAI have been obtained that are suitable for X-ray analysis at high resolution. 相似文献
163.
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165.
Membranes from unsaturated fatty acid auxotrophs of Escherichia coli were studied by spin labeling and freeze-fracturing. From measurements of the partition of the spin label TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) between the aqueous phase and fluid lipids in isolated membranes, temperatures, corresponding to the onset and completion of a lateral phase separation of the membrane phospholipids were determined. By freeze-fracture electron microscopy a change in the distribution of particle in the membrane was observed around the temperature of the onset of the lateral phase separation. When cells were frozen from above that temperature a netlike distribution of particles in the plasma membrane was observed for unfixed preparations. When frozen after fixing with glutaraldehyde the particle distribution was random. In membranes of cells frozen with or without fixing from a temperature below the onset of the phase separation, the particles were aggregated and large areas void of particles were present. This behavior can be understood in terms of the freezing rate with the aid of phase diagrams. 相似文献
166.
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169.
C S Buer K T Gahagan GA Swartzlander Jr P J Weathers 《Journal of industrial microbiology & biotechnology》1998,21(4-5):233-236
Comparison of the optical trapping efficiency of Agrobacterium rhizogenes and A. tumefaciens strains indicates the A. rhizogenes strain, ATCC 11325, is significantly less efficiently trapped than A. rhizogenes A4, ATCC 15834, and the A. tumefaciens strain LBA4404. Differences were also found in capsule generation, growth media viscosity, and transmission electron microscopy
negative staining. These observations imply a difference in surface structure exists. Calcofluor fluorescence suggests the
difference involves an exopolysaccharide.
Received 1 July 1998/ Accepted in revised form 5 October 1998 相似文献
170.
Kleemann DO Walker SK Walkley JR Smith DH Grimson RJ Stone BA Seamark RF 《Theriogenology》1989,31(6):1183-1190
The fertility enhancing effects of semen were examined following the intra-uterine insemination of killed spermatozoa plus seminal plasma 17 d prior to insemination with viable spermatozoa. Three experiments were conducted: two on 1.5-yr old and 2.5 to 5.5 yr-old Booroola Merino x South Australian Merino ewes in 1986 and one on 1.5 yr-old ewes in 1987. Differences between treatment and control groups for the percentage of ewes exhibiting estrus by Days 21 and 35 following fertile insemination, the percentage of ewes with viable embryos at Day 35, the number and weight of viable embryos per ewe, the nubmer of caruncular implantation sites and the progesterone level were not statistically significant (P>0.05). There were no statistically significant treatment by experiment interactions for any of the variables examined. Inflammation and edema of the endometrial tissue was not observed following the presensitization treatment. 相似文献