1,720,956 research outputs found
Regulation of Chitin Oligosaccharides Utilization in Escherichia Coli
Full text linkThe genome of Escherichia coli harbors several catabolic operons involved in the utilization of a wide variety of natural compounds as carbon sources. The chitobiose (chu) operons of E.coli Is involved in the utilization of chitobiose(disaccharide of N-acety1-D-glucosamine) and cellbiose (disaccharide of glucose) derived from the two most abundant naturally occurring carbon sources on earth, chitin and cellulose respectively. The operon consists of the chbBCARFG genes coding for transport, regulation and hydrolysis functions required to utilize these compounds; the chuyBCA genes code for a multi-subuni PTS transporter ; the chuR codes for a dual function repressor/activator of the operon; the chbF codes for a phospho-glucosidase and the chbG codes for a protein of unknown function.
The chu operon Is regulated by three transcription factors; NagC, a key regulator of the nag genes involved in amino sugar metabolism; ChbR, a dual function operon-specific regulator; and CRP_cAMP. The operon is repressed by NagC and ChbR in the absence of catabolic substrate. In the presence of chitobiose, expression is induced by the abrogation of NagC-mediated repression by GlcNAc-6-P generated by the hydrolysis of chitobiose-6-P and subsequent activation of transcription by ChbR and CPR-cAMP.
Wild type E.coli connot utilize cellbiose due to the inability of cellbiose to induce expression from the operon. The simultaneous presence of a loss of function mutation in nagC and a gain –of-function mutation in chbR is necessary and sufficient to allow cellbiose to induce expression and confer on E.coli the ability to utilize cellbiose.
The activation step by ChbR and CPR-cAMP requires an inducer that is recognized by ChbR. The chemical identity of the inducer and the mechanism of transcriptional activation by ChbR and CPR-cAMP are not understood.
The studies described in the chapter 2 shows that chbG is essential for the utilization of the acetylated sugars chitobiose and chitotriose while it is dispensable for the sugars lacking the acety1group such as cellobiose and chitosan dimer, a disaccharide of N-glucosamine. ChbG is produced as a cytosolic protein and removes one acety1 group from chitobiose and chitotriose thus shows a mono-decetylase activity. Taken together, the observing suggest that ChbG deacetylates chitobiose-6-P and chitotriose-6-P producing the mono-decetylated from of the sugars. The deacetylateion is necessary for their recognition both as inducers by ChbR to activate transcription along with CRP-cAMP and as substractes by phosop-glucosidase ChbF. Cellobiose positive(Cel+) mutants carrying nagC delection and different gain-of-function mutations in chbR are independent of chbG for induction by chitobiose suggesting that the mutations in ChbR can allow it to recognize the acetylated form of chitobiose-6-P. Despite normal induction, the mutants to grow on chitobiose without chbG are consistant with the requirement of deacetylation for hydrolysis by ChbF.
The prediction active site of chbG was validated by demonstrating the loss of chbG function upon alanine substitution of the putative metal binding residues. Vibro cholerace ChbG can complement the function of E.coli ChbG indicating that ChbG is conserved in both the organisms.
The studies presented in chapter 3 address the mechanism of transcriptional activation of the chb operon by ChbR and CPR-cAMP. ChbR and CPR-cAMP function in a synergistic manner in response to the induction signal. The synergy is not because of their cooperative binding to the DNA. The role of CRP as a class I activator via the known mechanism involving interaction between the Activation region1 (AR1) and the C-terminal domain of the alpha subunit of RNA polymerase (CTD) was not crucial for the chb operon. A direct interaction between the two activators in virto was observed. Based on these results and the close spacing of the synergy is due to interaction between the two regulators bound to DNA that is enhanced in the presence of the inducer, binding about an optimal confirmation in ChbR required to interact with RNA polymerase. ChbR contacts different residues in the subunit in response to cellbiose and chitobiose; whereas it utilizes the known residues in the presence cellbiose, it appears to require different and unknown residues for induction in the presence of chitobiose.
In conclusion, the studies reported in chapter 2 and 3 provide an understanding of the regulation of the chitin oligosaccharides utilization in E.coli at different levels. The broad implications of these studies and possible future directions are discussed in chapter 4. ChbG is an evolutionary conserved protein found in both prokaryotes and enkayotes including humans. ChbG homologs have been implicated in inflammatory bowel disorders in humans and development in metazoans. Therefore, the studies on chbG described in this thesis have been broader significance
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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