摘要:

AbstractStudies of the G2 to M transition in amphibian oocytes, in combination within vitromitotic systems and yeast genetic analysis, have significantly contributed to our understanding of the mechanisms by which M‐phase is regulated. Historically, oocyte maturation has provided a number of valuable initial observations, but the biochemical elucidation of cell cycle control mechanisms has proved more tractable in cell‐free extracts of frog eggs which reproduce aspects of early embryogenic mitosis. Recent experiments examining the importance of protein synthesis in the maturing oocyte have highlighted some important differences between mitosis and meiosis. Additional controls found in meiosis but not embryonic mitosis, are similar to controls found in somatic cells. This suggests that understanding the differences, as well as the similarities, between meiosis in the oocyte and mitosis in the early embryo will help us to learn more about the way in which cells enter and leave mito

ISSN:0265-9247    

DOI:10.1002/bies.950150302

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractSome metabolic pathways are nearly ubiquitous among organisms: the genes encoding the enzymes for such pathways must therefore be ancient and essential.De novopyrimidine biosynthesis is an example of one such metabolic pathway. In animals a single protein called CADAbbreviations: CAD, trifunctional protein catalyzing the first three steps ofde novopyrimidine biosynthesis in higher eukaryotes; CPS, carbamyl phosphate synthetase domain; CPSase, carbamyl phosphate synthetase activity; ATC, aspartate transcarbamylase domain; ATCase, aspartate transcarbamylase activity; DHO, dihydroorotase domain; DHOase, dihydroorotase activity; GLN, glutaminase subdomain or subunit of carbamyl phosphate synthetase, GL Nase, glutaminase activity; SYN, synthetase subdomain or subunit of carbamyl phosphate synthetase; SYNase, synthetase activity.carries the first three steps of this pathway. The same three enzymes in prokaryotes are associated with separate proteins. TheCADgene appears to have evolved through a process of gene duplication and DNA rearrangement, leading to an in‐frame gene fusion encoding a chimeric protein. A driving force for the creation of eukaryotic genes encoding multienzymatic proteins such as CAD may be the advantage of coordinate expression of enzymes catalyzing steps in a biosynthetic pathway. The analogous structure in bacteria is the operon. Differences in the translational mechanisms of eukaryotes and prokaryotes may have dictated the different strategies used by organisms to evolve coordinately regulated gene

ISSN:0265-9247    

DOI:10.1002/bies.950150303

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractNuclear pre‐mRNAs must be precisely processed to give rise to mature cytoplasmic mRNAs. This maturation process, known as splicing, involves excision of intron sequences and ligation of the exon sequences. One of the major problems in understanding this process is how splice sites, the sequences which form the boundaries between introns and exons, can be accurately selected. A number of studies have defined conserved sequences within introns which were later shown to interact with small nuclear ribonucleoproteins (snRNPs). However, due to the simplicity of these conserved sequences it has become clear that other elements must be involved and a number of studies have indicated the importance of secondary structures within pre‐mRNAs. Using various examples, we shall show that such structures can help to specify splice sites by modifying physical distances within introns or by being involved in the definition of exons and, lastly, that they can be part of the regulation of alternative splic

ISSN:0265-9247    

DOI:10.1002/bies.950150304

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractCell proliferation in response to growth factors is mediated by specific high affinity receptors. Ligand‐binding by receptors of the protein tyrosine kinase family results in the stimulation of several intracellular signal transduction pathways. Key signalling enzymes are recruited to the plasma membrane through the formation of stable complexes with activated receptors. These interactions are mediated by the conserved, non‐catalytic SH2 domains present in the signalling molecules, which bind with high affinity and specificity to tyrosine‐phosphorylated sequences on the receptors. The assembly of enzyme complexes is emerging as a major mechanism of signal transduction and may regulate the pleiotropic effects of growth fa

ISSN:0265-9247    

DOI:10.1002/bies.950150305

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractThevavproto‐oncogene encodes a 95 kDa protein which is expressed exclusively in hematopoietic cells. Analysis of the deduced amino acid sequence has revealed the presence of asrc‐homology 2 (SH2) domain, 2 SH3 domains, a cysteine‐rich region with similarity to protein kinase C, and a region highly similar to proteins with guanine nucleotide exchange activity onras‐like GTPases. Recent work has shown thatvavis tyrosine phosphorylated in response to stimulation of surface membrane receptors in a variety of hematopoietic cell lines.Vavmay play a role in hematopoietic cell signaling by coupling tyrosine kinase pathways toras‐like GTPases through the regulation of guanine nucleotide

ISSN:0265-9247    

DOI:10.1002/bies.950150306

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractThec‐ros, c‐metandc‐neugenes encode receptor‐type tyrosine kinases and were originally identified because of their oncogenic potential. However, recent progress in the analysis of these receptors and their respective ligands indicate that they do not mediate exclusively mitogenic signals. Rather, they can induce cell movement, differentiation or morphogenesis of epithelial cells in culture. Interestingly, the discussed receptors are expressed in embryonal epithelia, whereas direct and indirect evidence shows that the corresponding ligands are produced in mesenchymal cells. In development, signals given by mesenchymal cells are major driving forces for differentiation and morphogenesis of epithelia; embryonal epithelia are generally unable to differentiate without the appropriate mesenchymal factors. The observed activities of these receptor/ligand systems in cultured cells and their expression patterns indicate that they regulate epithelial differentiation and morphogenesis also during embryogenesis and suggest thus a molecular basis for mesenchymal epithelial inter

ISSN:0265-9247    

DOI:10.1002/bies.950150307

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractSkeletal muscle formation is studied in vitro with myogenic cell lines and primary muscle cell cultures, and in vivo with embryos of several species. We review several of the notable advances obtained from studies of cultured cells, including the recognition of myoblast diversity, isolation of the MyoD family of muscle regulatory factors, and identification of promoter elements required for muscle‐specific gene expression. These studies have led to the ideas that myoblast diversity underlies the formation of the multiple types of fast and slow muscle fibers, and that myogenesis is controlled by a combination of ubiquitous and muscle‐specific transcriptional regulators that may be different for each gene. We further review some unexpected results that have been obtained when ideas from work in culture have been tested in developing animals. The studies in vivo point to additional molecular and cellular mechanisms that regulate muscle formation in the ani

ISSN:0265-9247    

DOI:10.1002/bies.950150308

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

ISSN:0265-9247    

DOI:10.1002/bies.950150309

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractThe molecular signals that determine the position and timing of the furrow that forms during mammalian cellcytokinesis are presently unknown. It is apparent, however, that these signals are generated by the mitotic spindle after the onset ofanaphase. Recently we have described a structure that bisects the cell during telophase at the position of the cytokinetic furrow. Thisstructure, the telephase disc, appears to the templated by the motitc spindle during anaphase, and precedes the formation of the cytokinetic furrow. The relationship of the telephase disc to the myosin and actin based furrowing mechanism is discussed here. We propose that the telophase disc may determine the position and timing of cleavage by recruitment and alignment of myosin.

ISSN:0265-9247    

DOI:10.1002/bies.950150310

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractCells fine‐tune their DNA repair, selecting some regions of the genome in preference to others. In the paradigm case, excision of UV‐induced pyrimidine dimers in mammalian cells, repair is concentrated in transcribed genes, especially in the transcribed strand. This is due both to chromatin structure being looser in transcribing domains, allowing more rapid repair, and to repair enzymes being coupled to RNA polymerases stalled at damage sites; possibly other factors are also involved. Some repair‐defective diseases may involve repair‐transcription coupling: three candidate genes have been suggested.However, preferential excision of pyrimidine dimers is not uniformly linked to transcription. In mammals it varies with species, and with cell differentiation. InDrosophilaembryo cells it is absent, and in yeast, the determining factor is nucleosome stability rather than transcription.Repair of other damage departs further from the paradigm, even in some UV‐mimetic lesions. No selectivity is known for repair of the very frequent minor forms of base damage. And the most interesting consequence of selective repair, selective mutagenesis, normally occurs for UV‐induced, but not for spontaneous mutations. The temptation to extrapolate from mammalian UV repair should

ISSN:0265-9247    

DOI:10.1002/bies.950150311

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY