bluuebunny/crossref_metadata_2025_split huggingface dataset:
The extracellular matrix is instructive
Authors: Caplan Arnold I. 
Published: February 2005

AbstractThe extracellular matrix does more than just blanket cells; it also provides informational cues which affect a variety of developmental and cellular maintenance activities. The constituents of the matrix provide the fabric for cell motility and cell shape as well as anchorage sites for bioactive factors which directly affect the cell's developmental pattern or mitotic activity. The influence of the extracellular matrix is controlled by the cell's responsiveness to these complex signals. The same matrix component, for example hyaluronic acid, can have completely different effects depending on the cell's lineage and developmental history. The functional interaction between the extracellular matrix and specific cell surface receptors provides cues which affect the control of development and the maintenance and aging events that affect specific cells and tissues.
Models for MHC‐restricted T‐cell antigen recognition
Authors: Norcross Michael A. 
Published: February 2005

AbstractCurrent models for T‐cell recognition of foreign antigen depict the T‐cell receptor as having a single antibody‐like combining site which binds a complex of MHC and antigen. An alternative hypothesis is presented here; it is proposed that the first domains of the MHC function as inverted V‐like regions to complement the TcR V‐regions in creating antigen binding sites.
Regulation of immunoglobulin variable region gene assembly: Development of the primary antibody repertoire
Authors: Berman Jeffrey E. , Malynn Barbara A. , Blackwell T. Keith , Alt Frederick W. 
Published: February 2005

AbstractThe immune system can generate an almost infinite number of different antibody specificities, the sum of which is the antibody repertoire. This article considers aspects of the mechanism and control of immunoglobulin variable (V) region gene assembly with a focus on how these factors may affect generation of the antibody repertoire in normal and disease states. New model systems to study the mechanism and control of V gene assembly are described, in particular the introduction of V gene recombination substrates into Abelson murine leukemia virus‐transformed pre‐B cells. Finally, a model is presented which suggests that control of V gene assembly is mediated by regulating accessibility of substrate gene segments to recombinational machinery.
The Drosophila position‐specific antigens. Clues to their morphogenetic role
Authors: Leptin Maria , Wilcox Michael 
Published: February 2005

AbstractThe Drosophila position‐specific antigens are a family of cell‐surface glycoprotein complexes showing spatially restricted patterns of expression. Changes in these distributions correlate with morphogenetic events like compartment‐alization and the formation of grooves and folds during tissue organization. The complexes each contain a common component associated with different variable components. Different tissues, organs and regions of the body express complexes containing different subsets of variable components. The structure of the complexes resembles that of the family of vertebrate receptors for fibronectin, molecules which function in morphogenetic processes involving cell migration and attachment. It is possible that the position‐specific antigens have a similar role in fly development.
Autoregulation of tubulin synthesis
Authors: Caron Joan M. , Kirschner Marc W. 
Published: February 2005

AbstractIn many mammalian cell types, increases in the level of nonpolymerized tubulin cause an inhibition in tubulin synthesis which is accompanied by a decrease in tubulin mRNA levels. To see whether inhibition is caused by nuclear or cytoplasmic events, two groups have recently examined the ability of enucleated cells to autoregulate tubulin synthesis.1,2 These experiments have demonstrated that transcription, processing, and transport of tubulin mRNAs from the nucleus to the cytoplasm are not major sites of autoregulation. Instead, monomeric tubulin must reduce, either directly or indirectly, the translatability of its own message.
Control of the early activation genes of T lymphocytes
Authors: Crabtree Gerald R. , Durand David 
Published: February 2005

AbstractBinding of antigen or lectin to the surface of a T lymphocyte initiates a complex sequence of events which result in both T cell proliferation and the acquisition of immunologic functions. This complex sequence of events is most likely programmed and precisely timed by a series of contingent gene activations in which one member of this series activates the next. The two most obvious examples of these stages are the set of genes activated when antigen interacts with the antigen receptor and the set of genes activated when IL‐2 interacts with its receptor. However, it is likely that several other parallel pathways of gene activation are necessary to bring about T‐cell division. An example would be the antigen‐induced activation of the c‐myc gene, which in turn is likely to be involved in regulating an additional group of genes not directly controlled by either IL‐2 or antigen. A fundamental understanding of T‐lymphocyte activation necessitates identification of the factors involved in initiating each of these contingent levels of gene activation.
Discontinuous RNA synthesis through trans‐splicing
Authors: Braun Richard 
Published: February 2005

AbstractIn eukaryotic cells intron sequences are usually spliced out with a high degree of precision from heterogenous nuclear RNA (hnRNA) to give functional mRNA with exons in their right order. Provided with the right substrates, cell extracts can achieve the same. With exotic substrates, on the other hand, the same extracts can cut exons from one RNA and join them to exons from another RNA, a process termed trans‐splicing. In vivo, RNA trans‐splicing could lead to faulty, but also to novel proteins and, through reverse transcription, to genomic rearrangements. So far, in living cells trans‐splicing has only been invoked to occur in trypanosomes. In these unicellular organisms most if not all mRNAs are made discontinuously, so that the mature mRNAs carry, at their 5′ end, a common 35‐nucleotide sequence, called a miniexon, encoded separately in the genome. The miniexon is most likely joined to the body of the mRNA by trans‐splicing.
Coronavirus leader‐RNA‐primed transcription: An alternative mechanism to RNA splicing
Authors: Lai Michael M. C. 
Published: February 2005

AbstractMany viral and cellular mRNA species contain a leader sequence derived from a distant upstream site on the same gene by a process of RNA splicing. This process usually involves either nuclear functions or self‐splicing of RNA molecules. Coronavirus, a cytoplasmic RNA virus, unfolds yet another mechanism of joining RNA, which involves the use of a free leader RNA molecule. This molecule is synthesized and dissociates from the template RNA, and subsequently reassociates with the template RNA at down‐stream initiation sites of subgenomic mRNAs to serve as the primer for transcription. This leader‐primed transcriptional process thus generates viral mRNAs with a fused leader sequence. A similar mechanism might also operate in the mRNA transcription of African trypanosomes.
Viral sequences required for neurovirulence of poliovirus
Authors: Racaniello Vincent R. 
Published: February 2005

AbstractPolioviruses of reduced neurovirulence contain point mutations in the viral RNA that are responsible for the attenuated phenotype. Two such point mutations have been identified in the genomes of the Sabin live oral vaccine strains, one in the 5′‐noncoding region of the viral RNA, and one in capsid polypeptide VP3.
Prions are novel infectious pathogens causing scrapie and creutzfeldt—Jakob disease
Authors: Prusiner Stanley B. 
Published: February 2005

AbstractScrapie and Creutzfeldt–Jakob disease (CJD) are caused by prions, which appear to be different from both viruses and viroids. Prions contain protein which is required for infectivity, but no nucleic acid has been found within them. Prion proteins are encoded by a cellular gene and not by a nucleic acid within the infectious prion particle. A cellular homologue of the prion protein has been IDentified. The role of this homologue in metabolism is unknown. Prion proteins, but not the cellular homologue, aggregate into rod‐shaped particles that are histo‐chemically and ultrastructurally IDentical to amyloid. Extracellular collections of prion proteins form amyloid plaques in scrapie‐ and CJD‐infected rodent brains as well as CJD‐infected human brains. Within the plaques, prion proteins assemble to form amyloid filaments. Elucidating the molecular differences between the prion protein and its cellular homologue may be important in understanding the chemical structure and replication of prions.
Genes specifying cytokinin biosynthesis in prokaryotes
Authors: Morris Roy O. , Powell Gary K. 
Published: February 2005

AbstractCytokinins are plant hormones which have long been associated with cell division and plastid differentiation. Recently, they have been found to play a central role also in the growth of plant tumors. Certain phytopathogenic bacteria, notably Agrobacterium tumefaciens and Pseudomonas syringae pv. savastanoi, can incite tumors on dicotyledonous plants and such tumors exhibit growth which is characteristic of the presence of excess auxin and cytokinin. Genes specifying cytokinin biosynthesis have now been isolated from both sets of bacteria. The genes encode prenyl transferase responsible for cytokinin biosynthesis which, upon expression in E. coli,cause the production of the active cytokinin, zeatin. Expression of these genes in association with the plant is responsible for at least part of the tumor phenotype, although the molecular mechanisms of infection by these bacteria are apparently quite dissimilar. There is extensive homology between the cytokinin biosynthetic genes from the two sets of bacteria.
Processing and termination of RNA polymerase I transcripts
Authors: Reeder Ronald H. , Labhart Paul , McStay Brian 
Published: February 2005

AbstractElectron micrographs of active ribosomal genes from many species show a similar picture in which gene regions covered with nascent transcripts alternate with apparently non‐transcribed spacers. Since the gradients of visible nascent transcripts stop near the 3′ end of the 28S sequence it has often been assumed that transcription by RNA polymerase I also terminates at that point. Recent biochemical studies have shown however, that transcription continues far beyond the 3′ end of the 28S and in some species continues across the entire spacer. In this article we review the evidence for spacer transcription in Xenopus, mouse and Drosophila and discuss possible reasons for its invisibility in the electron microscope.
The zona pellucida: A coat of many colors
Authors: Wassarman Paul M. 
Published: February 2005

AbstractThe zona pellucida is an extracellular coat that surrounds all mammalian eggs. It is a porous matrix of interconnected filaments that are assembled from glycoproteins synthesized and secreted by growing oocytes. The zona pellucida is responsible both for species‐specific binding of sperm to unfertilized eggs and inducing bound sperm to undergo the acrosome reaction. The latter enables sperm to penetrate the extracellular coat and fertilize the egg. The zona pellucida also aids in prevention of polyspermy following fertilization and in protection of preimplantation embryos. In mice, several of these important functions can now be ascribed to specific zona pellucida glycoproteins that have been purified and characterized. Furthermore, the enzyme responsible for hatching of embryos from the zona pellucida, just prior to implantation, has been identified and characterized.
Mitogen‐specific nonresponsive mutants and the initiation of cellular proliferation
Authors: Herschman Harvey R. 
Published: February 2005

AbstractA large number of molecular, physiological, and biochemical events accompany the mitogen‐induced transition from a quiescent, nonproliferating state to a cycling state. However, it is difficult to distinguish causal steps in the commitment to cell division from correlative events. To approach this problem we have isolated mitogen‐specific nonproliferative variants of 3T3 cells, namely cell lines that are unable to mount a mitogenic response to specific mitogens.
Brain protein 4.1 subtypes: A working hypothesis
Authors: Krebs Keith E. , Zagon Ian S. , Sihag Ram , Goodman Steven R. 
Published: February 2005

AbstractIn a companion review1 we discussed the data supporting the conclusion that at least two subtypes of spectrin exist in mammalian brain. One form is found in the cell bodies, dendrites, and post‐synaptic terminals of neurons (brain spectrin(240/235E)) and the other subtype is located in the axons and presynaptic terminals (brain spectrin(240/235)). Our recent understanding of brain spectrin subtype localization suggests a possible explanation for a conundrum concerning brain 4.1 localization. Amelin, an immunoreactive analogue of red blood cell (rbc) cytoskeletal protein 4.1, is localized in neuronal cell bodies and dendrites when brain sections are stained with antibody against rbc protein 4.1. However, it has recently been suggested that synapsin I, a neuron‐specific phosphoprotein associated with the cytoplasmic surface of small synaptic vesicles, is related to erythrocyte 4.1. In this review we hypothesize that there are at least two forms of brain 4.1: a cell body/dendritic form (amelin) which is detected with rbc protein 4.1 antibody, and a unique form found exclusively in the presynaptic terminal (synapsin I). The binding of synapsin I to brain spectrin(240/235), and its ability to stimulate the spectrin/F‐actin interaction in a phosphorylation‐dependent manner suggests a model for the regulation of synaptic transmission mediated by the neuronal cytoskeleton.
Genetics and molecular biology of rhythms
Authors: Hall Jeffrey C. , Rosbash Michael 
Published: February 2005

AbstractMutations that disrupt biological rhythms have existed in microbial and metazoan eukaryotes for some time. They have recently begun to be studied with increasing intensity, both in terms of phenotypic effects of the relevant genetic variants, and with regard to molecular isolation and analysis of the genes defined by two of the ‘clock mutations’. These genetic loci, called period (per) in Drosophila and frequency (frq) in Neurospora, influence not only the basic characteristics of circadian rhythmicity, but also temperature compensation of such daily cycle durations, ‘re‐setting’ of the rhythms' phases, and (for the per mutants) behavioral oscillations associated with much shorter than circadian periodicities. Molecular cloning of Drosophila's per gene, accompanied crucially by the locus's identification in germ‐line transformants, has led to information on its expression ‐ temporally, spatially, and in regard to heterogeneity of mRNA types. Nucleotide‐sequencing analyses of genomic DNA (and/or cDNA) from normal and mutated per alleles have (1) led to the suggestion that this clock gene encodes a family of proteoglycans (which was further indicated by application of antibody reagents obtained by manipulation of one of the gene's exons); and (2) shown that the three types of per mutations ‐ which shorten or lengthen rhythm periodicities or appear to eliminate them ‐ are associated with interesting amino‐acid substitutions or a stop codon, respectively. Analogous molecular findings are awaited from Neurospora, whose frq gene has very recently been cloned and definitively identified, in part via transformation experiments.
Regulation of drug‐metabolizing enzymes during the perinatal period in rat and human liver
Authors: Cresteil Thierry 
Published: February 2005

AbstractThe importance of drug‐metabolizing enzymes in developing mammals has been recently reevaluated in view of the activities and potential inducibilities of these enzymes. The role of endogenous factors raises the question of whether there is a positive regulation of the expression of drug‐metabolizing enzymes by hormones. In humans, among the different isoenzymes of cytochrome P‐450 described in adult liver, only one is absent in 20‐week‐old fetuses. Epoxide hydrolase and glutathione S‐transferases are active while UDP‐glucuronidation develops postnatally. The consequence of this asynchronous rise of activities is briefly discussed.
Topological aspects of site‐specific DNA‐inversion
Authors: Kanaar Roland , van de Putte Pieter 
Published: February 2005

AbstractSite‐specific recombination events are of fundamental importance in many biological systems. In vitro experiments using purified proteins and DNA sub‐strates are yielding insights into strand exchange mechanisms and synapsis of recombination sites. By examining results across a range of systems ‐ prokaryotic and eukaryotic ‐ two distinct classes of site‐specific recombinase enzymes can be defined.
Probing the structure and function of viral RNA genomes
Authors: Nuss Donald L. , Banerjee Amiya K. 
Published: February 2005

AbstractThe majority of human, animal and plant viral pathogens possess genomes composed of RNA. The strategies evolved for expression and replication of viral RNA genomes can differ significantly from those utilized for expression and replication of host‐cell genetic material. Consequently, knowledge of the molecular details of these strategies can lead to a clearer understanding of the origin, evolution and control of viral pathogens. We describe recent progress in identifying important structural and functional domains of the RNA genomes and associated replicative enzymes for two very different viruses: vesicular stomatitis virus, which possesses a single‐stranded RNA genome of negative polarity, and wound tumor virus, which contains a genome composed of 12 discrete segments of double‐stranded RNA.
Evolution of haemoglobin studied by protein engineering
Authors: Nagai Kiyoshi , Luisi Ben , Shih Daniel 
Published: February 2005

AbstractVertebrate haemoglobin (Hb), the oxygen‐carrying protein of the blood, consists of two α‐ and two β‐subunits, each containing one haem, and shows cooperative oxygen binding known as the haem–haem interaction. The amino‐acid sequences of Hbs found in different species have diverged considerably and the homology in the most distantly related ones is only 40%. How can such varied amino‐acid sequences give rise to similar three‐dimensional structures and functional properties? To what extent do amino‐acid replacements affect the structure of haemoglobin? Which mutations are responsible for the functional differences between haemoglobins from different species? Expression of α‐ and β‐globins in Escherichia coli has enabled us to introduce any mutations in the haemoglobin molecule at will. By X‐ray crystallographic and functional studies of the engineered mutants, we are trying to answer these questions.
Mesodermal determination genes: Evidence from DNA methylation studies
Authors: Harrington Maureen A. , Jones Peter A. 
Published: February 2005

AbstractMouse embryo cells, primed to differentiate with the hypomethylating agent 5‐azacytidine (5‐aza‐CR), provide an excellent model system in which cellular differentiation can be studied at the molecular level. An inherent advantage of this system is the availability of clonal populations of cells representative of the non‐differentiated precursor, those whose determinative state is that of a specific lineage, and the end stage, phenotypically mature cell. Analysis of these cultures at the cellular and molecular level will advance our understanding of requirements for and the mechanism of action of growth modulators as well as the necessity for controlled expression of certain proto‐oncogenes. At a more fundamental level, the system can be used to identify, isolate and characterize genes whose expression alters the phenotypic fate of cells.
A family of closely related ATP‐binding subunits from prokaryotic and eukaryotic cells
Authors: Higgins Christopher F. , Gallagher Maurice P. , Mimmack Michael L. , Pearce Stephen R. 
Published: February 2005

AbstractA large number of cellular proteins bind ATP, frequently utilizing the free energy of ATP hydrolysis to drive specific biological reactions. Recently, a family of closely related ATP‐binding proteins has been identified, the members of which share considerable sequence identity. These proteins, from both prokaryotic and eukaryotic sources, presumably had a common evolutionary origin and include the product of the white locus of Drosophila, the P‐glycoprotein which confers multidrug resistance on mammalian tumours, and prokaryotic proteins associated with such diverse processes as membrane transport, cell division, nodulation and DNA repair. A comparison of these various proteins provides valuable insights into the function and evolution of the multicomponent systems with which they are associated.
Facts and hypotheses concerning the function of non‐granulated cells in the adenohypophysis of vertebrates
Authors: Harrisson Fernand 
Published: February 2005

AbstractElectron microscopic examination of the non‐granulated cells of the adenohypophysis of several species has evidenced a unity of structure amongst vertebrates. Efforts to correlate structure and function have been made, but they have often been hampered by the scarcity or absence of direct experimental methods for the investigation of their function. Yet often the evidence in favor of a given role has been circumstantial, relying on coincidences between changes in the secretory activity of the gland and changes in the ultrastructure of the non‐granulated cells or in colloid production. At the present time, the prevailing hypothesis is that, in addition to a possible role in mechanical support within the gland, the non‐granulated cells seem to be important with respect to the secretory process of the endocrine cells and/or to the metabolism of the adenohypophysis.
Initiation of eukaryotic DNA replication in vitro
Authors: Stillman Bruce 
Published: February 2005

AbstractRecent advances in our understanding of the mechanism and regulation of eukaryotic DNA replication have been expedited by the use of cell‐free systems capable of initiation of DNA replication. The system capable of replicating plasmid DNAs containing the SV40 origin of DNA replication in vitro is a paradigm for studies on the replication of other virus DNAs and the replication of cellular chromosomes. This review outlines some of the contemporary issues and developments related to this complex problem.
Recombination in the eukaryotic nucleus
Authors: Hastings P. J. 
Published: February 2005

AbstractMitotic recombination is a repair process which is known to repair double strand breaks and to fill double‐strand gaps by copying a homologous sequence. Meiotic recombination is a process of heteroduplex formation which sometimes generates crossovers. Evidence is presented that the later stages of meiotic recombination have some characteristics of mitotic repair recombination, leading to the conclusion that mismatch repair may be a recombinogenic repair process. The evidence suggests that the recombinational repair process generates hetero‐duplex bubbles which can move. Some bubbles become crossovers. Others cease to exist, perhaps because topoisomerase activity breaks them down.
Mosaic evolution in the Drosophila genome
Authors: Martin Christopher H. , Meyerowitz Elliot M. 
Published: February 2005

AbstractInvestigation of the evolution of the DNA sequences that comprise a small gene cluster in five closely related species of Drosophila shows that it evolves as a mosaic, with adjacent subregions displaying very different rates and types of evolution. It appears that local differences in the influence of selectional and mutational forces result in the formation of these distinct subregions.