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M Hoek

Publications and source records attributed to M Hoek.

14 recordsLinked to original sources
PubMed2006

Double-hadron leptoproduction in the nuclear medium.

The first measurements of double-hadron production in deep-inelastic scattering within the nuclear medium were made with the HERMES spectrometer at DESY HERA using a 27.6 GeV positron beam. By comparing data for deuterium, nitrogen, krypton, and xenon nuclei, the influence of the nuclear medium on the ratio of double-hadron to single-hadron yields was investigated. Nuclear effects on the additional hadron are clearly observed, but with little or no difference among nitrogen, krypton, or xenon, and with smaller magnitude than effects seen on previously measured single-hadron multiplicities. The data are compared with models based on partonic energy loss or prehadronic scattering and with a model based on a purely absorptive treatment of the final-state interactions. Thus, the double-hadron ratio provides an additional tool for studying modifications of hadronization in nuclear matter.

Journal Article↗
PubMed2005

Measurement of the tensor structure function b1 of the deuteron.

The Hermes experiment has investigated the tensor spin structure of the deuteron using the 27.6 GeV/c positron beam of DESY HERA. The use of a tensor-polarized deuteron gas target with only a negligible residual vector polarization enabled the first measurement of the tensor asymmetry A(d)zz and the tensor structure function b(d)1 for average values of the Bjorken variable 0.01< <0.45 and of the negative of the squared four-momentum transfer 0.5 GeV2 < <5 GeV2. The quantities A(d)zz and b(d)1 are found to be nonzero. The rise of b(d)1 for decreasing values of x can be interpreted to originate from the same mechanism that leads to nuclear shadowing in unpolarized scattering.

Journal Article↗
PubMed2005

Single-spin asymmetries in semi-inclusive deep-inelastic scattering on a transversely polarized hydrogen target.

Single-spin asymmetries for semi-inclusive electroproduction of charged pions in deep-inelastic scattering of positrons are measured for the first time with transverse target polarization. The asymmetry depends on the azimuthal angles of both the pion (phi) and the target spin axis (phi(S)) about the virtual-photon direction and relative to the lepton scattering plane. The extracted Fourier component sin((phi+phi(S))(pi)(UT) is a signal of the previously unmeasured quark transversity distribution, in conjunction with the Collins fragmentation function, also unknown. The component sin((phi-phi(S)(pi)(UT) arises from a correlation between the transverse polarization of the target nucleon and the intrinsic transverse momentum of quarks, as represented by the previously unmeasured Sivers distribution function. Evidence for both signals is observed, but the Sivers asymmetry may be affected by exclusive vector meson production.

Journal Article↗
PubMed2004

Flavor decomposition of the sea-quark helicity distributions in the nucleon from semiinclusive deep inelastic scattering.

Double-spin asymmetries of semiinclusive cross sections for the production of identified pions and kaons have been measured in deep inelastic scattering of polarized positrons on a polarized deuterium target. Five helicity distributions including those for three sea quark flavors were extracted from these data together with reanalyzed previous data for identified pions from a hydrogen target. These distributions are consistent with zero for all three sea flavors. A recently predicted flavor asymmetry in the polarization of the light quark sea appears to be disfavored by the data.

Journal Article↗
PubMed2004

Bioavailability of selenium from fish, yeast and selenate: a comparative study in humans using stable isotopes.

OBJECTIVE: To measure the bioavailability of selenium from cooked and raw fish in humans by estimating and comparing apparent absorption and retention of selenium in biosynthetically labelled fish with labelled selenate and biosynthetically labelled selenium in brewers yeast. DESIGN: The intervention study was a parallel, randomised, reference substance controlled design carried out at two different centres in Europe. SETTING: The human study was carried out at the Institute of Food Research, Norwich, UK and at TNO Nutrition and Food Research, Zeist, The Netherlands. SUBJECTS: In all, 35 male volunteers aged 18-50 y were recruited; 17 subjects were studied in Norwich (UK) and 18 in Zeist (Netherlands). All of the recruited subjects completed the study. INTERVENTIONS: Biosynthetically labelled trout fish (processed by two different methods), biosynthetically labelled brewers yeast and isotopically labelled selenate were used to estimate selenium apparent absorption and retention by quantitative analysis of stable isotope labels recovered in faeces and urine. Subjects consumed the labelled foods in four meals over two consecutive days and absorption was measured by the luminal disappearance method over 10 days. Urinary clearance of isotopic labels was measured over 7 days to enable retention to be calculated. RESULTS: Apparent absorption of selenium from fish was similar to selenate and there was no difference between the two processing methods used. However, retention of fish selenium was significantly higher than selenate (P<0.001). Apparent absorption and retention of yeast selenium was significantly different (P<0.001) from both fish selenium and selenate. CONCLUSION: Fish selenium is a highly bioavailable source of dietary selenium. Cooking did not affect selenium apparent absorption or retention from fish. Selenium from yeast is less bioavailable.

Adolescent↗
PubMed2001

Large-insert BAC/YAC libraries for selective re-isolation of genomic regions by homologous recombination in yeast.

We constructed representative large-insert bacterial artificial chromosome (BAC) libraries of two human pathogens (Trypanosoma brucei and Giardia lamblia) using a new hybrid vector, pTARBAC1, containing a yeast artificial chromosome (YAC) cassette (a yeast selectable marker and a centromere). The cassette allows transferring of BACs into yeast for their further modification. Furthermore, the new hybrid vector provides the opportunity to re-isolate each DNA insert without construction of a new library of random clones. Digestion of a BAC DNA by an endonuclease that has no recognition site in the vector, but which deletes most of the internal insert sequence and leaves the unique flanking sequences, converts a BAC into a TAR vector, thus allowing direct gene isolation. Cotransformation of a TAR vector and genomic DNA into yeast spheroplasts, and subsequent recombination between the TAR vector's flanking ends and a specific genomic fragment, allows rescue of the fragment as a circular YAC/BAC molecule. Here we prove a new cloning strategy by re-isolation of randomly chosen genomic fragments of different size from T. brucei cloned in BACs. We conclude that genomic regions of unicellular eukaryotes can be easily re-isolated using this technique, which provides an opportunity to study evolution of these genomes and the role of genome instability in pathogenicity.

Animals↗
PubMed2000

Expression-site-associated gene 8 (ESAG8) of Trypanosoma brucei is apparently essential and accumulates in the nucleolus.

Trypanosoma brucei variant surface glycoprotein expression sites are interesting examples of genomic loci under complex epigenetic control. In the infectious bloodstream stage, only one of about 20 expression sites is actively transcribed. In the Tsetse midgut (procyclic) stage, chromatin remodeling silences all expression sites. We have begun to explore the function of one of the expression-site-associated genes, ESAG8. Gene knockout experiments implied that ESAG8 is essential. ESAG8 is present at a very low level and apparently accumulates in the nucleolus. A 32-amino-acid domain, which contains a putative bipartite nuclear localization signal (NLS), is both necessary and sufficient to target fusions of ESAG8, with Aequorea victoria green fluorescent protein, to the trypanosome nucleolus. This same sequence functioned only as an NLS in mammalian cells, supporting the idea that nucleolar accumulation requires specific interactions. These results have implications for models of ESAG8 function.

Amino Acid Sequence↗
PubMed1998

Regulated processive transcription of chromatin by T7 RNA polymerase in Trypanosoma brucei.

Inability of T7 RNA polymerase to processively transcribe higher eukaryotic chromatin is interpreted as a correlate of its reported inhibition by nucleosomes on reconstituted templates in vitro . We used chromosomally integrated reporter cassettes to examine features of T7 transcription in a lower eukaryotic system. Luciferase reporters were targeted to rDNA in transgenic Trypanosoma brucei stably expressing the phage polymerase. Because trypanosome mRNAs are capped by RNA splicing in trans , T7 transcription could be gauged by luciferase activity. In contrast to findings from higher eukaryotes, T7 transcription is vigorous and processive on chromatin templates in T.brucei , surpassing levels achieved with endogenous promoters, including those recruiting RNA polymerase I. This may be a reflection of intrinsic differences in chromatin structure between differently evolved eukaryotes or of an integration site that is exceptionally permissive for T7 transcription due to a local accessible chromatin conformation. T7 transcription could be manipulated to achieve different levels of constitutive expression, through the use of promoter mutations. Moreover, T7 initiation could be regulated by the prokaryotic Tet repressor and elongation halted by T7 terminator sequences. We have exploited these features to construct a robust inducible expression system, whose utility potentially extends to other trans -splicing organisms.

Animals↗
PubMed1994

Replication in cultured C2C12 muscle cells correlates with the neuroinvasiveness of California serogroup bunyaviruses.

The neuroinvasiveness of California serogroup bunyaviruses is determined by the ability of the virus to replicate in striated muscle after peripheral inoculation of mice. Neuroinvasiveness was mapped to the medium (M) RNA segment of the virus, which encodes the viral glycoproteins, when reassortants were made between La Crosse/original virus, a neuroinvasive isolate, and Tahyna-181/57 virus, a nonneuroinvasive clone. We have tested the murine muscle cell line C2C12 as a surrogate for myotropism and have found that there is a slight, but reproducible difference in the replication of virus clones bearing the M RNA segment of La Crosse/original virus compared to clones bearing the M RNA segment of Tahyna-181/57 virus, as determined by viral titer, antigen expression, and plaque formation.

Animals↗
PubMed1994

Role of Swi4 in cell cycle regulation of CLN2 expression.

Expression of the Saccharomyces cerevisiae CLN1 and CLN2 genes is cell cycle regulated, and the genes may be controlled by positive feedback. It has been proposed that positive feedback operates via Cln/Cdc28 activation of the Swi4/Swi6 transcription factor, leading to CLN1 and CLN2 transcription due to Swi4 binding to specific sites (SCBs) in the CLN1 and CLN2 promoters. To test this proposal, we have examined the effects of deletion either of the potential SCBs in the CLN2 promoter or of the SWI4 gene on CLN2 transcriptional control. Deletion of a restriction fragment containing the identified SCBs from the promoter does not prevent cell cycle regulation of CLN2 expression, although expression is lowered at all cell cycle positions. A promoter containing a 5.5-kb plasmid insertion or an independent 2.5-kb insertion at the point of deletion of the SCB-containing restriction fragment also exhibits cell cycle regulation, so involvement of unidentified upstream SCBs is unlikely. Neither Swi4 nor the related Mbp1 transcription factor is required for cell cycle regulation of the intact CLN2 promoter. In contrast, Swi4 (but not Mbp1) is required for correct cell cycle regulation of the insertion/deletion promoter lacking SCB sites. We have extended previous genetic evidence for involvement of Swi4 in some aspect of CLN2 function: a mutant hunt for CLN2 positive regulatory factors yielded only swi4 mutations at saturation. Swi4 may bind to nonconsensus sequences in the CLN2 promoter (possibly in addition to consensus sites), or it may act indirectly to regulate CLN2 expression.

Alleles↗