摘要:

AbstractOver 50 years have passed since Roger Sperry formulated a simple model of how visual space, as seen by the retina, can be projected onto the brain in a two‐dimensional, topographic map during development. Sperry posited a set of two orthogonal gradients in the retina that gives each cell a positional identity. He further suggested that these molecules could be used to match up with complementary gradients in the target field of the retinal projection, the tectum. While some investigators hold that the existence of such molecules may not be necessary to establish retinotectal maps, recent work has identified several cell surface proteins whose distributions are of the type predicted by Sperry. An unexpected twist comes from culture assays demonstrating that inhibitory activities on tectal membranes can guide the growth of processes from retinal neurons. Moreover, the expression patterns of several enzymes and three transcription factors suggest that these proteins are candidates for regulatory agents in the determination of cell position in the retina. In addition, results from perturbation experiments support the candidacy of two of the enzymes, and a new mutant screen has uncovered several as yet unidentified genes that are required for establishment of the proper retinotectal map. A number of these results were presented at a recent meeting on neurospecificity held in Cargese, Corsica and sponsored by NATO and NS

ISSN:0265-9247    

DOI:10.1002/bies.950160102

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

年代:1994

数据来源: WILEY

摘要:

AbstractDNA helicases catalyze the disruption of the hydrogen bonds that hold the two strands of double‐stranded DNA together. This energy‐requiring unwinding reaction results in the formation of the single‐stranded DNA required as a template or reaction intermediate in DNA replication, repair and recombination. A combination of biochemical and genetic studies have been used to probe and define the roles of the multiple DNA helicases found inE. coli.This work and similar efforts in eukaryotic cells, although far from complete, have established that DNA helicases are essential components of the machinery that interacts with the DNA mol

ISSN:0265-9247    

DOI:10.1002/bies.950160103

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

年代:1994

数据来源: WILEY

摘要:

AbstractDifferent families of transport proteins catalyze transmembrane solute translocation, employing different mechanisms and energy sources. Several of these functionally dissimilar proteins nevertheless exhibit similar strutural units, consisting of six tightly packed α‐helices which may comprise all or part of a transmembrane channel. It is now recognized that some of these families arose independently of each other by convergence, while others arose from common precursors by divergence. The former families apparently arose at different times in evolutionary history, in different groups of organisms, employing different rout

ISSN:0265-9247    

DOI:10.1002/bies.950160104

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

年代:1994

数据来源: WILEY

摘要:

AbstractA major obstacel to the study of mammalian development, and to the practical application of knowledge gained from it in the clinic during therapeuticin vitrofertilisation and embryo transfer (IVF‐ET), is the propensity of embryos to become retarded or arrested during their culturein vitro.The precise developmental cell cycle in which embryos arrest or delay is characteristic for the species and coincides with the earliest period of embryonic gene expression. Much evidence reviewed here implicates free oxygen radicals (FORs) in the process of arrest. Thus, studies on the development of mouse preimplantation embryosin vitrohave shown that (i) FORs are elevatedin vitro, but notin vivo, at the time at which embryos become arrested or delayed, (ii) systems for removing reactive oxygen species to limit the formation of hydroxy radicals are present, although they have not yet been assessed quantitatively and may differ qualitatively from those in adult cells, (iii) metabolic and possibly genetic adaptations to oxidative damage are evident, (iv) published procedures for overcomingin vitroarrest are explicable in terms of FOR‐mediated damage or responses and (v) the arrest or delay of most embryosin vitrocan be reduced or prevented experimentally by addition of metal chelators to limit hydroxy radical formation and lipid hydroperoxidat

ISSN:0265-9247    

DOI:10.1002/bies.950160105

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

年代:1994

数据来源: WILEY

摘要:

AbstractPhosphoinositides are inositol‐containing phospholipids whose hydrolysis is a key step in the rapid responses of animal cells to extracellular signals. Whether they play similar roles in plant cells has not been established, and some have suggested alternative roles as direct modulators of specific proteins. Nonetheless, evidence is accumulating that phosphoinositide hydrolysis mediates transduction of some signal in plants. The evidence is strongest for a role in triggering the shedding of flagella by the unicellular algaChlamydomonas reinhardtiiunder acid stress. Rapid kinetic analysis indicates that phosphoinositide hydrolysis occures within half a second and could trigger the rapid loss of flagella. Plant responses to pathogens and osmotic stress, as well as the regulation of turgor changes which underlie stomatal opening and closing and the movement of leaves and flower parts, may also be mediated by phosphoinositide hydrolysis. The evidence thus indicates that at least one reason plants have phosphoinositides is to mediate transduction of environmental signal

ISSN:0265-9247    

DOI:10.1002/bies.950160106

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

年代:1994

数据来源: WILEY

摘要:

AbstractPaxillin is a recently identified member of the complex of cytoskeletal proteins that is found concentrated in cultured cells andin vivoat the cytoplasmic face of regions of cell attachment to the extracellular matrix. These sites, in view of their close proximity to the extracellular matrix, are well positioned to act as signal‐transducing centers to ‘report on’ changes in the cells, immediate environment. Recent findings indicate that such signals are in part mediated through the activation of tyrosine kinases concentrated at the sites of adhesion. Changes in the phosphotyrosine content of paxillin accompanying this elevation in kinase activity suggest that paxillin may be an important intermediary in these pat

ISSN:0265-9247    

DOI:10.1002/bies.950160107

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

年代:1994

数据来源: WILEY

ISSN:0265-9247    

DOI:10.1002/bies.950160108

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

年代:1994

数据来源: WILEY

ISSN:0265-9247    

DOI:10.1002/bies.950160109

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

年代:1994

数据来源: WILEY

摘要:

AbstractExtensive studies of DNA secondary structure during the past decade have shown that DNA is a dynamic molecule, whose structure depends on the underlying nucleotide sequence and is influenced by the environment and the overall DNA topology. Three major non‐B‐DNA structures have been described (Z‐DNA, triplex DNA and cruciform DNA) which are stabilized by unconstrained negative supercoiling and can be formed under physiological conditions. In this essay we summarize the DNA primary structure features that are pertinent to the formation of these conformers and present data concerning the occurrence of these sequences in the eukaryotic genome. The evidence in favor of the existence of these unusual DNA structuresin vivois discussed. The effect of alternative non‐B‐DNA structures on the way DNA is organized in chromatin is considered, and this is followed by evaluation of the data relating these structures to eukaryotic transcription. Some possible mechanisms by which the effect of non‐B structures on transcription might be exerted a

ISSN:0265-9247    

DOI:10.1002/bies.950160110

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

年代:1994

数据来源: WILEY

ISSN:0265-9247    

DOI:10.1002/bies.950160111

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

年代:1994

数据来源: WILEY