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Key words: Genome evolution — Adaptability — Somatic hypermutation — Affinity maturation (1)
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Biologie (4)

Digitale Medien

Enhanced Evolvability in Immunoglobulin V Genes Under Somatic Hypermutation (1999)

Cowell, Lindsay G. ; Kim, Hye-Jung ; Humaljoki, Tiina ; [weitere]

Springer

Journal of molecular evolution 49 (1999), S. 23-26

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ISSN: 1432-1432

Schlagwort(e): Key words: Genome evolution — Adaptability — Somatic hypermutation — Affinity maturation

Quelle: Springer Online Journal Archives 1860-2000

Thema: Biologie

Notizen: Abstract. Darwinian theory requires that mutations be produced in a nonanticipatory manner; it is nonetheless consistent to suggest that mutations that have repeatedly led to nonviable phenotypes would be introduced less frequently than others—if under appropriate genetic control. Immunoglobulins produced during infection acquire point mutations that are subsequently selected for improved binding to the eliciting antigen. We and others have speculated that an enhancement of mutability in the complementarity-determining regions (CDR; where mutations have a greater chance of being advantageous) and/or decrement of mutability in the framework regions (FR; where mutations are more likely to be lethal) may be accomplished by differential codon usage in concert with the known sequence specificity of the hypermutation mechanism. We have examined 115 nonproductively rearranged human Ig sequences. The mutation patterns in these unexpressed genes are unselected and therefore directly reflect inherent mutation biases. Using a χ2 test, we have shown that the number of mutations in the CDRs is significantly higher than the number of mutations found in the FRs, providing direct evidence for the hypothesis that mutations are preferentially targeted into the CDRs.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/PL00006530

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Digitale Medien

Al3+-Ca2+ interactions in aluminum rhizotoxicity (1993)

Kinraide, Thomas B. ; Ryan, Peter R. ; Kochian, Leon V.

Springer

Planta 192 (1993), S. 104-109

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ISSN: 1432-2048

Schlagwort(e): Aluminum toxicity ; Calcium displacement ; Electrical potential ; Root ; Triticum

Quelle: Springer Online Journal Archives 1860-2000

Thema: Biologie

Notizen: Abstract Several mineral rhizotoxicities, including those induced by Al3+, H+, and Na+, can be relieved by elevated Ca2+ in the rooting medium. This leads to the hypothesis that the toxic cations displace Ca2+ from transport channels or surface ligands that must be occupied by Ca2+ in order for root elongation to occur. In this study with wheat (Triticum aestivum L.) seedlings, we have determined, in the case of Al3+, that (i) Ca2+, Mg2+, and Sr2+ are equally ameliorative, (ii) that root elongation does not increase as Ca2+ replaces Mg2+ or Sr2+ in the rooting media, and (iii) that rhizotoxicity is a function solely of Al3+ activity at the root-cell membrane surface as computed by a Gouy-Chapman-Stern model. The rhizotoxicity was indifferent to the computed membrane-surface Ca2+ activity. The rhizotoxicity induced by high levels of tris(ethylenediamine)cobaltic ion (TEC3+), in contrast to Al3+, was specifically relieved by Ca2+ at the membrane surface. The rhizotoxicity induced by H+ exhibited a weak specific response to Ca2+ at the membrane surface. We conclude that the Ca2+-displacement hypothesis fails in the case of Al3+ rhizotoxicity and that amelioration by cations (including monovalent cations) occurs because of decreased membrane-surface negativity and the consequent decrease in the membrane-surface activity of Al3+. However, TEC3+, but not Al3+, may be toxic because it inhibits Ca2+ uptake. The nature of the specific H+-Ca2+ interaction is uncertain.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/BF00198699

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Digitale Medien

Al3+-Ca2+ interactions in aluminum rhizotoxicity (1993)

Ryan, Peter R. ; Kinraide, Thomas B. ; Kochian, Leon V.

Springer

Planta 192 (1993), S. 98-103

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ISSN: 1432-2048

Schlagwort(e): Aluminum toxicity ; Calcium uptake ; Growth inhibition ; Root ; Triticum

Quelle: Springer Online Journal Archives 1860-2000

Thema: Biologie

Notizen: Abstract The cation Al3+ is toxic to plants at micromolar concentrations and can severely inhibit root growth in solution experiments. Trivalent aluminum hydrolyzes in solution, and, apart from the Al3+ ion, which dominates speciation below pH 5.0, various mononuclear and polynuclear hydroxy-Al species can also occur (Kinraide 1991). Accumulating evidence suggests that Al3+ is the rhizotoxic species under the experimental conditions used in the present study (Kinraide 1991; Kinraide et al. 1992). The inhibition of Ca2+ uptake in roots by Al3+ has been proposed as a possible mechanism for Al3+ toxicity, and in this study the hypothesis was tested directly. Root growth and Ca2+ uptake were measured in 5-d-old seedlings of wheat (Triticum aestivum L. Thell) during exposure to Al3+ in a low-Ca2+ basal medium, and to Al3+ in the presence of added cations. Uptake of Ca2+ in whole roots and translocation to the shoot were measured using 45Ca2+, and localized measurements of net Ca2+ flux were also made at the root apex using the technique of microelectrode ion-flux estimation. Treatment with 2.64 μM AlCl3 in 226 μM CaCl2, at pH 4.5, severely inhibited root growth without affecting Ca2+ uptake. Addition of 30 mM Na2+, 3 mM Mg2+ or 50 μM tris(ethylenediamine)cobalt(III) to this Al3+ treatment restored root growth but significantly reduced Ca2+ uptake measured over the entire root system and at the root apex. The Al3+ and Ca2+ concentrations were adjusted so that the activities of the Al3+ and Ca2+ ions were constant in all solutions (1.5 μM and 200 μM, respectively). Root growth can be severely inhibited by Al3+ concentrations that do not affect Ca2+ uptake, while the addition of ameliorating cations depresses Ca2+ uptake. These results argue against the hypothesis that Al3+ inhibits root growth by reducing Ca2+ uptake.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/BF00198698

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Digitale Medien

Effects of reforestation methods on genetic diversity of lodgepole pine: an assessment using microsatellite and randomly amplified polymorphic DNA markers (1999)

Thomas, B. R. ; Macdonald, S. E. ; Hicks, M. ; [weitere]

Springer

Theoretical and applied genetics 98 (1999), S. 793-801

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ISSN: 1432-2242

Schlagwort(e): Key words Pinus contorta ; Silviculture ; Reforestation ; Gene conservation ; RAPD ; SSR ; DNA analyses

Quelle: Springer Online Journal Archives 1860-2000

Thema: Biologie

Notizen: Abstract We examined the effects of different methods of forest regeneration on the genetic diversity of lodgepole pine (Pinus contorta var ‘latifolia’) using two different DNA-based molecular markers [randomly amplified polymorphic DNA (RAPDs) and microsatellites or simple sequence repeats (SSRs)]. Genetic diversity was estimated for 30 individuals in each of four populations for the following three stand types: (1) mature lodgepole pine (〉100 years); (2) 20- to 30-year-old harvested stands left for natural regeneration; (3) 20- to 30-year-old planted stands (4 stands of each type); and one group of 30 operationally produced seedlings. There was no significant effect of stand type on expected heterozygosity, although allelic richness and diversity were much higher for SSRs than for RAPDs. Expected heterozygosity ranged from 0.39 to 0.47 based on RAPDs and from 0.67 to 0.77 based on SSRs. The number of alleles per locus for SSRs ranged from 3 to 34 (mean 21.0), and there was a significant relationship between sequence repeat length and the number of alleles at a locus. Both marker types showed that over 94% of the variation was contained within the populations and that the naturally regenerated stands sampled had lower (not significant) expected heterozygosity than the planted or unharvested stands. The group of seedlings (assessed by RAPDs only) had expected heterozygosity and allele frequencies similar to those of the unharvested stands. Genetic distance measures were higher than obtained previously in the species using isozyme markers. There was no correlation between the two marker types for pair-wise genetic distances based on populations analyzed by both methods. Pair-wise genetic distance measures and an ordination of allele frequencies for both marker types showed little effect of geographic location or stand type on genetic similarity.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/s001220051136

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