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  • 1
    ISSN: 1617-4623
    Keywords: Alfalfa ; Glutamine synthetase ; Gene structure ; Gene amplification
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary One of the members of the small gene family encoding glutamine synthetase in alfalfa (Medicago sativa L.) has been cloned and its nucleotide sequence determined. Approximately full length cDNAs corresponding to the gene have also been cloned and their sequences determined. The gene is about 4 kb long, and consists of twelve exons (size range 37 to 159 bp) and eleven introns (size range 90 to 715 bp). The messenger RNA contains a 120 nucleotide 5′-untranslated region, a 1,071 nucleotide coding region, and a 210 nucleotide 3′-untranslated region. Several eukaryotic consensus transcription and processing signals are present in this plant gene: (1) a TATA box (TATAAATA) 25 bp 5′ to the putative transcription start site; (2) a potential polyadenylation signal (AATAATAA) 20 bp 5′ to the polyadenylation site; and (3) the terminal dinucleotides of the introns (5′-GT...AG-3′). Also the dinucleotide, CpG, is significantly underrepresented throughout the gene. The derived amino acid sequence predicts a relatively hydrophilic protein with a molecular weight of 39,215. Several regions of amino acid homology are apparent when this sequence is compared to the Anabaena and Chinese hamster glutamine synthetase sequences. Using data derived from the gene structure, the minimum size of a glutamine synthetase gene amplification unit in an l-phosphinothricin resistant alfalfa tissue culture line has been determined to be 35 kb.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1520-510X
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    FEMS microbiology letters 153 (1997), S. 0 
    ISSN: 1574-6968
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Gas vesicles are buoyant intracellular organelles composed of a rigid proteinaceous membrane surrounding a gas-filled space. Many prokaryotic microorganisms including photosynthetic and heterotrophic bacteria and halophilic and methanogenic archaea produce gas vesicles. In the majority of cases gas vesicles function in providing vertical motility to cells in aquatic environments. Recent genetic analysis of several halophilic archaeal (haloarchaeal) species has shown that a large cluster of genes [gvp MLKJIHGFEDACN(O)] is necessary for gas vesicle formation.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: Transcription from the bop promoter in the haloarchaeon Halobacterium NRC-1, is highly induced under oxygen-limiting conditions. A DNA gyrase inhibitor, novobiocin, was previously shown to block bop gene induction and suggested that DNA supercoiling mediates transcriptional induction. A region of non-B structure was found 3′ to the TATA box within an 11 bp alternating purine–pyrimidine sequence (RY box), which correlated to both increased DNA supercoiling and transcriptional induction. Here, saturation mutagenesis of the RY box region has been used to show that single-base substitutions of A(r)G either 23 or 19 bp 5′ to the transcription start site temper the effect of DNA supercoiling based on novobiocin insensitivity of transcription. Mutagenesis of the region 5′ to the TATA box showed its involvement in DNA supercoiling modulation of transcription, defined the 3′ end of the upstream activator sequence (UAS) regulatory element, and ruled out the requirement for a TFB (TFIIB) Recognition Element. Spacing between the TATA box and UAS was found to be critical for promoter activity because insertion of partial or whole helical turns between the two elements completely inhibited transcription indicating that the UAS element does not function as a transcriptional enhancer. The results are discussed in the context of DNA melting and flexibility around the TATA box region and the involvement of multiple regulatory and transcription factors in bop promoter activity.
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  • 6
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: To facilitate the functional genomic analysis of an archaeon, we have developed a homologous gene replacement strategy for Halobacterium salinarum based on ura3, which encodes the pyrimidine biosynthetic enzyme orotidine-5′-monophosphate decarboxylase. H. salinarum was shown to be sensitive to 5-fluoroorotic acid (5-FOA), which can select for mutations in ura3. A spontaneous 5-FOA-resistant mutant was found to contain an insertion in ura3 and was a uracil auxotroph. Integration of ura3 at the bacterioopsin locus (bop ) of this mutant restored 5-FOA sensitivity and uracil prototrophy. Parallel results were obtained with a Δura3 strain constructed by gene replacement and with derivatives of this strain in which ura3 replaced bop. These results show that H. salinarum ura3 encodes functional orotidine-5′-monophosphate decarboxylase. To demonstrate ura3-based gene replacement, a Δbop strain was constructed by transforming a Δura3 host with a bop deletion plasmid containing a mevinolin resistance marker. In one approach, the host contained intact ura3 at the chromosomal bop locus; in another, ura3 was included in the plasmid. Plasmid integrants selected with mevinolin were resolved with 5-FOA, yielding Δbop recombinants at a frequency of 〉 10−2 in both approaches. These studies establish an efficient new genetic strategy towards the systematic knockout of genes in an archaeon.
    Type of Medium: Electronic Resource
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  • 7
    Publication Date: 2019-07-13
    Description: In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required to maintain short-lived biogenic gases as atmospheric signatures. We focus particularly on advances made since the seminal review by Des Marais et al. The purpose of this work is not to propose new biosignature strategies, a goal left to companion articles in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN56547 , Astrobiology (ISSN 1531-1074) (e-ISSN 1557-8070); 18; 6; 663-708
    Format: text
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  • 8
    Publication Date: 2019-07-12
    Description: For the first time in human history, we will soon be able to apply to the scientific method to the question "Are We Alone?" The rapid advance of exoplanet discovery, planetary systems science, and telescope technology will soon allow scientists to search for life beyond our Solar System through direct observation of extrasolar planets. This endeavor will occur alongside searches for habitable environments and signs of life within our Solar System. While these searches are thematically related and will inform each other, they will require separate observational techniques. The search for life on exoplanets holds potential through the great diversity of worlds to be explored beyond our Solar System. However, there are also unique challenges related to the relatively limited data this search will obtain on any individual world.
    Keywords: Lunar and Planetary Science and Exploration
    Type: GSFC-E-DAA-TN52771
    Format: application/pdf
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  • 9
    Publication Date: 2019-07-13
    Description: We introduce a Bayesian method for guiding future directions for detection of life on exoplanets. We describe empirical and theoretical work necessary to place constraints on the relevant likelihoods, including those emerging from better understanding stellar environment, planetary climate and geophysics, geochemical cycling, the universalities of physics and chemistry, the contingencies of evolutionary history, the properties of life as an emergent complex system, and the mechanisms driving the emergence of life. We provide examples for how the Bayesian formalism could guide future search strategies, including determining observations to prioritize or deciding between targeted searches or larger lower resolution surveys to generate ensemble statistics and address how a Bayesian methodology could constrain the prior probability of life with or without a positive detection.
    Keywords: Exobiology
    Type: GSFC-E-DAA-TN58405 , Astrobiology (ISSN 1531-1074) (e-ISSN 1557-8070); 18; 6; 779–824
    Format: text
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  • 10
    Publication Date: 2019-05-15
    Description: Here we review how environmental context can be used to interpret whether O2 is a biosignature in extrasolar planetary observations. This paper builds on the overview of current biosignature research discussed in Schwieterman et al. (2017), and provides an in-depth, interdisciplinary example of biosignature identification and observation that serves as a basis for the development of the general framework for biosignature assessment described in Catling et al., (2017). O2 is a potentially strong biosignature that was originally thought to be an unambiguous indicator for life at high-abundance. In exploring O2 as a biosignature, we describe the coevolution of life with the early Earth's environment, and how the interplay of sources and sinks in the planetary environment may have resulted in suppression of O2 release into the atmosphere for several billion years, a false negative for biologically generated O2. False positives may also be possible, with recent research showing potential mechanisms in exoplanet environments that may generate relatively high abundances of atmospheric O2 without a biosphere being present. These studies suggest that planetary characteristics that may enhance false negatives should be considered when selecting targets for biosignature searches. Similarly our ability to interpret O2 observed in an exoplanetary atmosphere is also crucially dependent on environmental context to rule out false positive mechanisms. We describe future photometric, spectroscopic and time-dependent observations of O2 and the planetary environment that could increase our confidence that any observed O2 is a biosignature, and help discriminate it from potential false positives. The rich, interdisciplinary study of O2 illustrates how a synthesis of our understanding of life's evolution and the early Earth, scientific computer modeling of star-planet interactions and predictive observations can enhance our understanding of biosignatures and guide and inform the development of next-generation planet detection and characterization missions. By observing and understanding O2 in its planetary context we can increase our confidence in the remote detection of life, and provide a model for biosignature development for other proposed biosignatures.
    Keywords: Exobiology
    Type: GSFC-E-DAA-TN58082 , Astrobiology (ISSN 1531-1074) (e-ISSN 1557-8070); 18; 6; 630-662
    Format: application/pdf
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