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Biomedical subjects

L Boone

Publications and source records attributed to L Boone.

12 recordsLinked to original sources
PubMed2001

Physiological responses to mixing in large scale bioreactors.

Escherichia coli fed-batch cultivations at 22 m3 scale were compared to corresponding laboratory scale processes and cultivations using a scale-down reactor furnished with a high-glucose concentration zone to mimic the conditions in a feed zone of the large bioreactor. Formate accumulated in the large reactor, indicating the existence of oxygen limitation zones. It is suggested that the reduced biomass yield at large scale partly is due to repeated production/re-assimilation of acetate from overflow metabolism and mixed acid fermentation products due to local moving zones with oxygen limitation. The conditions that generated mixed-acid fermentation in the scale-down reactor also induced a number of stress responses, monitored by analysis of mRNA of selected stress induced genes. The stress responses were relaxed when the cells returned to the substrate limited and oxygen sufficient compartment of the reactor. Corresponding analysis in the large reactor showed that the concentration of mRNA of four stress induced genes was lowest at the sampling port most distant from the feed zone. It is assumed that repeated induction/relaxation of stress responses in a large bioreactor may contribute to altered physiological properties of the cells grown in large-scale bioreactor. Flow cytometric analysis revealed reduced damage with respect to cytoplasmic membrane potential and integrity in cells grown in the dynamic environments of the large scale reactor and the scale-down reactor.

Acetic Acid↗
PubMed2000

At the maize/Agrobacterium interface: natural factors limiting host transformation.

BACKGROUND: Agrobacterium tumefaciens has been successfully harnessed as the only natural vector for the incorporation of foreign genes into higher plants, but its use in the grain crops is often limited. Low transformation efficiency has been partly attributed to a failure in the initial events in the transformation process, specifically in the capacity of the VirA/VirG two-component system to induce expression of the virulence genes. RESULTS: Here we show that the root exudate of Zea mays seedlings specifically inhibits virulence gene expression, determine that 2-hydroxy-4,7-dimethoxybenzoxazin-3-one (MDIBOA), which constitutes > 98% of the organic exudate of the roots of these seedlings, is the most potent and specific inhibitor of signal perception in A. tumefaciens-mediated gene transfer yet discovered, and develop a model that is able to predict the MDIBOA concentration at any distance from the root surface. Finally, variants of A. tumefaciens resistant to MDIBOA-mediated inhibition of vir gene expression have been selected and partially characterized. CONCLUSIONS: These results suggest a strategy in which a plant may resist pathogen invasion by specifically blocking virulence gene activation and yet ensure that the 'resistance factor' does not accumulate to levels sufficient to impose toxicity and selection pressure on the pathogen. The data further establish that naturally occurring inhibitors directed against signal perception by the VirA/VirG two-component regulatory system can play an important role in host defense. Finally, selected variants resistant to specific MDIBOA inhibition may now be used to extend the transformation efficiency of maize and possibly other cereals.

Agrobacterium tumefaciens↗
PubMed1998

On becoming a parasite: evaluating the role of wall oxidases in parasitic plant development.

BACKGROUND: The temporal and spatial control of the transition from vegetative to parasitic growth is critical to any parasite, but is essential to the sessile parasitic plants. It has been proposed that this transition in Striga spp. is controlled simply by an exuded oxidase that converts host cell-surface phenols into benzoquinones which act as developmental signals that mediate the transition. An understanding of this mechanism may identify the critical molecular events that made possible the evolution of parasitism in plants. RESULTS: PoxA and PoxB are identified as the only apoplastic phenol oxidases in Striga asiatica seedlings, and the genes encoding them have been cloned and sequenced. These peroxidase enzymes are capable of oxidizing the 60 known inducing phenols into a small set of benzoquinones, and it is these quinones that induce parasitic development. Analysis of the reaction requirements and comparisons to host enzymes, however, lead us to argue that PoxA and PoxB are not necessary for host recognition. CONCLUSIONS: A new model is proposed where constitutive production of an activated oxygen species (in the case of Striga, H2O2) mediates host recognition. This strategy would allow a parasite to exploit abundant host enzymes to produce the diffusible recognition signals by converting a standard host defense into a parasitic offense.

Amino Acid Sequence↗
PubMed1998

Recombinant human CXC-chemokine receptor-4 in melanophores are linked to Gi protein: seven transmembrane coreceptors for human immunodeficiency virus entry into cells.

This article describes the transient expression of the CXC chemokine receptor-4 in Xenopus laevis melanophores and the resulting functional assay for the endogenous ligand for this receptor stromal cell-derived factor (SDF)-1alpha. Specifically, it will be shown that SDF-1alpha produces increased light transmittance in transfected cells that is consistent with the activation of Gi protein. This stimulus pathway is further implicated by the abolition of this response after pretreatment of the cells with pertussis toxin, a known method for the inactivation of Gi protein. The fact that SDF-1alpha does not produce responses in nontransfected cells and that treatment of the cells with 12G5, an antibody specific for the CXC chemokine receptor-4, eliminates this response indicates that this ligand produces responses by activation of this receptor in these cells. The possible relevance to human immunodeficiency virus (HIV) entry into cells was explored by observing the effects of SDF-1alpha on HIV-mediated cell fusion. It was found that SDF-1alpha blocked cell-to-cell fusion (as has been previously reported) at concentrations 1200-fold greater than those required to produce Gi protein mediated responses. The implications of the functional assay to screening for new drugs to block HIV-mediated fusion is discussed.

Amino Acid Sequence↗
PubMed1985

5-year follow-up study of children with low-to-moderate lead absorption: electrophysiological evaluation.

Forty-nine children aged 6 to 12 years were evaluated for residual effects of lead exposure using psychometric, electrophysiological, and medical tests 5 years after initial assessment. The original range of blood lead (PbB) levels was 6-59 (mean = 28) micrograms/dl; the current range was 6-30 (mean = 14) micrograms/dl. A linear relationship between PbB and slow brain wave voltage during sensory conditioning was observed at initial evaluation and at 2-year follow-up. No significant relationship between PbB and slow wave voltage during passive conditioning was found at the 5-year follow-up, although a linear increase in slow wave negativity relative to the current PbB level during active conditioning was suggested by exploratory analyses. Another exploratory analysis revealed a significant linear relationship between the original PbB levels and the latency of waves III and V of the brainstem auditory evoked potential. The latency of both waves increased as a function of original PbB. Increased latency of these waves is suggestive of subclinical pathology of the auditory pathway rostral to the cochlear nucleus, although end-organ impairment cannot be ruled out. No threshold for the effect of Pb on auditory function was apparent.

Brain↗
PubMed1982

Genetic recombination in avian retroviruses.

The avian retroviruses--and probably other retroviruses as well--undergo a variety of recombinational events with relatively high efficiency. An understanding of the molecular basis of these events should provide insight into the important biological properties these agents exhibit when they become integrated into somatic or germ-line host cells, when they exchange genetic information among themselves, or when they transduce host cell genes. In this article we review molecular models for homologous recombination, against a background of the other types of recombination events that are typical of these viruses. It seems probable that the retroviruses will provide useful models for analysis of a variety of DNA rearrangements known to occur in eukaryotic cells.

Animals↗
PubMed1980

Isolation and characterization of recombinant DNA clones of avian retroviruses: size heterogeneity and instability of the direct repeat.

Unintegrated proviral DNA of Schmidt-Ruppin B Rous sarcoma virus was cloned in the bacteriophage lambda vector Charon 21A. A total of 12 independent recombinant lambda SRBtd clones which were derived from the transformation-defective component in the viral preparation were analyzed with restriction endonucleases and molecular hybridization techniques. Three classes of clones were observed. Type I clones contained a 5.0-megadalton insert of viral DNA, type II clones contained phage with two size classes of inserts (5.0 and 5.2 megadaltons), and one type III clone contained only a 5.2-megadalton insert. The smaller insert present in type II clones appeared to be derived by deletion of one copy of a directly repeated sequence which was present in the larger insert. Mapping data indicated that the deletion includes all or part of the terminal repeat found in linear double-stranded proviral DNA. Similar results were obtained from lambda RAV2 recombinant clones derived from Rous-associated virus type 2. Analysis of DNA from type II and type III clones of lambda SRBtd and lambda RAV2 revealed limited heterogeneity in the size of the direct repeat.

Avian Leukosis Virus↗