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排序方式: 共有181条查询结果,搜索用时 15 毫秒
1.
A mutant strain AA-522, temperature-sensitive for protein synthesis, was isolated from a stringent strain (CP-78) of Escherichia coli K-12. The mutant strain has a relaxed phenotype at the nonpermissive growth temperature. Protein synthesis stops completely at 42 C, whereas the rate of ribonucleic acid (RNA) synthesis is maintained at 20% of the 30 C rate. Sucrose-gradient centrifugation analysis of RNA-containing particles formed at 42 C indicated the presence of “relaxed particles.” These particles possess 16S and 23S RNA and are precursors to normal 50S and 30S ribosomal subunits. A search for the temperature-sensitive protein responsible for the halt in protein synthesis implicated phenylalanyl transfer RNA (tRNA) synthetase. Essentially no enzyme activity is detected in vitro at 30 or 40 C. Analysis of phenylalanyl tRNA synthetase activity in revertants of strain AA-522 indicated the presence of intragenic suppressor mutations. Revertants of strain AA-522 analyzed for the relaxed response at 42 C were all stringent; strain AA-522 was stringent at 30 C. These data indicate that a single mutation in phenylalanyl tRNA synthetase is responsible for both a block in protein synthesis and the relaxed phenotype at 42 C. 相似文献
2.
The ATP-binding-cassette transmembrane transporters (ABC transporters)
known from vertebrates belong to four major subfamilies: (1) the P-
glycoproteins (Pgp); (2) the cystic fibrosis transmembrane conductance
regulators (CFTR); (3) the Tap proteins encoded with the major
histocompatibility complex of mammals; and (4) the peroxisomal membrane
proteins. Both Pgp and CFTR have a structure suggesting a past internal
gene duplication; a phylogenetic analysis indicated that these duplications
occurred independently, while an independent tandem gene duplication
occurred in the case of the Tap family. Both the Pgp and Tap proteins show
evidence of relationship to bacterial ABC transporters lacking internal
duplication, and both are significantly more closely related to the HlyB
and MsbA families of transporters from purple bacteria than they are to ABC
transporters from nonpurple bacteria. The simplest hypothesis to explain
this observation is that eukaryotic Pgp and Tap genes are descended from a
mitochondrial gene or genes that were subsequently translocated to the
nuclear genome. The Pgp genes of eukaryotes are characterized by a
remarkable degree of convergent evolution between the ATP-binding cassettes
of their N- terminal and C-terminal halves, whereas no such convergence is
seen between the two halves of CFTR genes or between the duplicated Tap
genes. Exon 13 of the CFTR gene, which encodes a putative regulatory domain
not found in other ABC transporters apart from CFTR, showed high levels of
both synonymous and nonsynonymous difference in comparisons among different
mammalian species, suggesting that this region is a mutational hot spot.
相似文献
3.
4.
X Xiao G Hintermann AL Demanin J Piret 《Journal of industrial microbiology & biotechnology》1996,16(4):261-262
Streptomyces glaucescens is shown to possess -lactamase activity which is inhibitable by clavulanate. This is important in regard to its use as a cloning host for enzymes of \-lactam biosynthesis. 相似文献
5.
The modified organ culture of rat embryonic shields provides favorable conditions during 2 weeks for the differentiation of main tissue types. Since the terminal differentiation in explants is inferior to that obtained in the homografts of the same shields under the kidney capsule, we tried to improve the culture medium by adding some known regulatory molecules: db-cAMP, db-cGMP, ATP, AMP, and butyric acid. These agents were added to the liquid medium in the concentration of 1 mM. In the first part of the study the explants were fixed and weighed after 8 or 14 days in vitro culture, and histological sections were examined. When the explants were treated with db-cAMP during the second week of culture, the skeletal muscle appeared more frequently in the treated series than in controls, and the weight of the treated explants was sometimes increased when compared with the control series. The db-cGMP had no effect on differentiation, but stimulated the growth of the explants when applied during the first week of culture. On the contrary, the db-cAMP when added during the first week, severely impeded the growth of explants. Other agents seem to be ineffective. In the second part, the content of cAMP and cGMP was measured in normal explants. The radioimmunoassay showed the same content of cAMP and cGMP during the entire culture period. In the third part of our study the incorporation of tritiated uridine and tritiated thymidine was measured during the second week of culture after the addition of db-cAMP. During the first days of treatment with db-cAMP the uptake of tritiated uridine and thymidine was inhibited, whereas on the seventh day the uptake was similar to that of the control. We can conclude that both cyclic nucleotides have a visible effect on growth whereas only cAMP has a positive impact on the differentiation of myotubes in cultured rat embryonic shields. 相似文献
6.
7.
ERIC S. BONO PATRICK SMOLINSKI AL CASAGRANDA JUNDE XU 《Computer methods in biomechanics and biomedical engineering》2013,16(2):125-131
The Shear-slip Mesh Update Method (SSMUM) is being used in flow simulations involving large but regular displacements of one or more boundaries of the computational domain. We follow up the earlier discussion of the method with notes on practical implementation aspects. In order to establish a benchmark problem for this class of flow problems, we define and report results from a two-dimensional viscous flow around a rotating stirrer in a square chamber. The application potential of the method is demonstrated in the context of biomedical design problem, as we perform an analysis of blood flow in a centrifugal left ventricular assist device, or blood pump, which involves a rotating impeller in a non-axisymmetric housing. 相似文献
8.
9.
HENRY SILVERMAN BABIKER AHMED SAMAR AJEILET SUMAIA AL‐FADIL SUHAIL AL‐AMAD HADIR EL‐DESSOUKY IBRAHIM EL‐GENDY MOHAMED EL‐GUINDI MUSTAFA EL‐NIMEIRI RANA MUZAFFAR AZZA SALEH 《Developing world bioethics》2010,10(2):70-77
To help ensure the ethical conduct of research, many have recommended educational efforts in research ethics to investigators and members of research ethics committees (RECs). One type of education activity involves multi‐day workshops in research ethics. To be effective, such workshops should contain the appropriate content and teaching techniques geared towards the learning styles of the targeted audiences. To ensure consistency in content and quality, we describe the development of a curriculum guide, core competencies and associated learning objectives and activities to help educators organize research ethics workshops in their respective institutions. The curriculum guide is divided into modular units to enable planners to develop workshops of different lengths and choose content materials that match the needs, abilities, and prior experiences of the target audiences. The content material in the curriculum guide is relevant for audiences in the Middle East, because individuals from the Middle East who participated in a Certificate Program in research ethics selected and developed the training materials (e.g., articles, powerpoint slides, case studies, protocols). Also, many of the activities incorporate active‐learning methods, consisting of group work activities analyzing case studies and reviewing protocols. The development of such a workshop training curriculum guide represents a sustainable educational resource to enhance research ethics capacity in the Middle East. 相似文献
10.
Phylogenetic tests of the hypothesis of block duplication of homologous genes on human chromosomes 6, 9, and 1 总被引:8,自引:1,他引:7
There are 10 gene families that have members on both human chromosome 6
(6p21.3, the location of the human major histocompatibility complex [MHC])
and human chromosome 9 (mostly 9q33-34). Six of these families also have
members on mouse chromosome 17 (the mouse MHC chromosome) and mouse
chromosome 2. In addition, four of these families have members on human
chromosome 1 (1q21-25 and 1p13), and two of these have members on mouse
chromosome 1. One hypothesis to explain these patterns is that members of
the 10 gene families of human chromosomes 6 and 9 were duplicated
simultaneously as a result of polyploidization or duplication of a
chromosome segment ("block duplication"). A subsequent block duplication
has been proposed to account for the presence of representatives of four of
these families on human chromosome 1. Phylogenetic analyses of the 9 gene
families for which data were available decisively rejected the hypothesis
of block duplication as an overall explanation of these patterns. Three to
five of the genes on human chromosomes 6 and 9 probably duplicated
simultaneously early in vertebrate history, prior to the divergence of
jawed and jawless vertebrates, and shortly after that, all four of the
genes on chromosomes 1 and 9 probably duplicated as a block. However, the
other genes duplicated at different times scattered over at least 1.6
billion years. Since the occurrence of these clusters of related genes
cannot be explained by block duplication, one alternative explanation is
that they cluster together because of shared functional characteristics
relating to expression patterns.
相似文献