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291.
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
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Several and various types of cells contain fine cytoplasmic filaments closely resembling the myofilaments of muscle cells (2, 18, 23, 24). In many of these cells and especially when cultured, it has been demonstrated that some of these filaments react with heavy meromyosin (HMM) in the same way as do the actin filaments of muscle cells (3, 6 7). This suggests that these filaments may be actinoid and form part of a contractile system. As fine intracytoplasmic filaments do occur in lymphatic endothelial cells (2, 14), we undertook an electron microscope investigation of their fine structure and their reaction on incubation with HMM and EDTA. We postulated that lymphatic endothelial cells possess a contractile filamentous system to which these filaments belong. 相似文献
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Mice of the TO Swiss strain received diets containing different amounts of saturated or unsaturated fat throughout life. These diets produced characteristic changes in cardiac phospholipid fatty acid composition, but produced no significant differences in fatty acid composition of phospholipids from a crude membrane fraction of brain. When littermates of these animals were exposed to ethanol vapour in an inhalation chamber it was observed that mice which had received a diet high in saturated fat lost the righting reflex at an estimated concentration of ethanol in blood higher than that required for mice receiving a control diet, or a diet rich in polyunsaturated fat. Analysis of the brain membrane fraction from those animals which had received ethanol revealed that mice receiving the highly saturated fat diet now had a significantly greater proportion of saturated fatty acids in brain membrane phospholipids. These results are discussed in relation to the hypothesis that brain membrane lipid composition may influence the behavioural response to ethanol. 相似文献
298.
An experimental system was developed in which the majority of all lymphocyte cell-surface proteins, regardless of antigenic specificity, could be cross-linked and redistributed in the membrane to determine whether this would induce a corresponding redistribution of intramembrane particles (IMP). Mouse spleen cells were treated with P-diazoniumphenyl- β-D-lactoside (lac) to modify all exposed cell-surface proteins. Extensive azo- coupling was achieved without significantly reducing cell viability or compromising cellular function in mitogen- or antigen-stimulated cultures. When the lac-modified cell- surface proteins were capped with a sandwich of rabbit antilactoside antibody and fluorescein-goat anti-rabbit Ig, freeze-fracture preparations obtained from these cells revealed no obvious redistribution of IMP on the majority of fracture faces. However, detailed analysis showed a statistically significant 35 percent decrease (P less than 0.01) in average IMP density in the E face of the lac-capped spleen cells compared with control cells, whereas a few E-face micrographs showed intense IMP aggregation. In contrast, there was no significant alteration of P-face IMP densities or distribution. Apparently, the majority of E-face IMP and virtually all P-face IMP densities or distribution. Apparently, the majority of E-face IMP and virtually all P-face IMP do not present accessible antigenic sites on the lymphocyte surface and do not associate in a stable manner with surface protein antigens. This finding suggests that IMP, as observed in freeze-fracture analysis, may not comprise a representative reflection of lymphocyte transmembrane protein molecules and complexes because other evidence establishes: (a) that at least some common lymphocyte surface antigens are indeed exposed portions of transmembrane proteins and (b) that the aggregation of molecules of any surface antigen results in altered organization of contractile proteins at the cytoplasmic face of the membrane. 相似文献
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Luisa María Sierra Albert Pastink Madeleine JM Nivard Ekkehart W. Vogel 《Molecular & general genetics : MGG》1993,237(3):370-374
Summary The DNA base sequence changes induced by diethyl sulfate (DES) were analyzed in postmeiotic male germ cells of Drosophila melanogaster. 31 transmissible vermilion mutants were recovered in F1 and F2 generations, with a frequency of 2.6 × 10–4 for the F1, and of 1.8–13 × 10–4 for the F2. The results show that DES induces both base pair substitutions (93%) and deletions (7%). In accord with its relatively high ability to alkylate oxygens in DNA, the most frequent type of sequence alteration among the basepair changes are GC-AT transitions, accounting for 73% of mutations, followed by transversions AT-TA (10%). DES also induced AT-GC transitions and AT-CG transversions. Both induced deletions were intralocus deletions, not occurring between basepair repeats. No influence of neighboring bases on the mutation position was found. 相似文献