Biomedical subjects

H Johannesson

Publications and source records attributed to H Johannesson.

16 recordsLinked to original sources

PubMed2006

Phase-specific gene expression underlying morphological adaptations of the dimorphic human pathogenic fungus, Coccidioides posadasii.

Coccidioides posadasii is a dimorphic fungal pathogen that grows as a filamentous saprobe in the soil and as endosporulating spherules within the host. To identify genes specific to the pathogenic phase of Co. posadasii, we carried out a large-scale study of gene expression in two isolates of the species. From the sequenced Co. posadasii genome, we chose 1,000 open reading frames to construct a 70-mer microarray. RNA was recovered from both isolates at three life-cycle phases: hyphae, presegmented spherules, and spherules releasing endospores. Comparative hybridizations were conducted in a circuit design, permitting comparison between both isolates at all three life-cycle phases, and among all life-cycle phases for each isolate. By using this approach, we identified 92 genes that were differentially expressed between pathogenic and saprobic phases in both fungal isolates, and 43 genes with consistent differential expression between the two parasitic developmental phases. Genes with elevated expression in the pathogenic phases of both isolates included a number of genes that were involved in the response to environmental stress as well as in the metabolism of lipids. The latter observation is in agreement with previous studies demonstrating that spherules contain a higher proportion of lipids than saprobic phase tissue. Intriguingly, we discovered statistically significant and divergent levels of gene expression between the two isolates profiled for 64 genes. The results suggest that incorporating more than one isolate in the experimental design offers a means of categorizing the large collection of candidate genes that transcriptional profiling typically identifies into those that are strain-specific and those that characterize the entire species.

Adaptation, Biological↗

PubMed2003

Parental tracking in the postfire wood decay ascomycete Daldinia loculata using highly variable nuclear gene loci.

The origin of the male and female gametes involved in fertilization events within a local population of the postfire wood decay ascomycete Daldinia loculata was investigated by genotyping the mycelia growing in the wood and the sexual ascospores, using three highly variable nuclear gene loci. The study was conducted in a geographically isolated burned forest site in southern Sweden. An intensive sampling was performed by collecting stromata containing ascospores and wood samples containing mycelia. In total, from 32 mapped burned birches, cultures of 22 haploid genets from decayed wood and six ascospores from each of 19 stromata were isolated and analysed. In 80% of the investigated burned branches, only one genet was found. From the analysis of the ascospore genotypes, we detected 30 fertilization events and 60% of them were the result of mating between conidia (clonal propagules) acting as male gametes and the genets in the branches representing the female gametes. The male parents producing the conidia were detected within the same local population as the female parents in 27% of the fertilization events and originated either from the same branch or from different trees located at 0.5-36 m away from the female parents. In 33% of the fertilization events, conidia originated from three male parents that were not found within the local population sampled. These parents could be anywhere inside or outside the sampled area. For the remaining fertilization events, we could not rule out the ascospores or the conidia as fertilizing propagules. No strong evidence for fertilization by recombinant propagules (ascospores) was detected in this study. The pyrophilous insect species associated with conidia of D. loculata are suggested to be essential vectors for the realization of the sexual cycle of this fungal species. By feeding on the conidia and flying between nearby trees inhabiting wood decay mycelia, these insects allow the transfer of conidia and therefore the opposite mating types to meet within a localized burned forest site.

Ascomycota↗

PubMed2001

Kondo resonance in a mesoscopic ring coupled to a quantum dot: exact results for the Aharonov-Bohm-Casher effects.

We study the persistent currents induced by both the Aharonov-Bohm and Aharonov-Casher effects in a one-dimensional mesoscopic ring coupled to a sidebranch quantum dot at Kondo resonance. For privileged values of the Aharonov-Bohm-Casher fluxes, the problem can be mapped onto an integrable model, exactly solvable by a Bethe ansatz. In the case of a pure magnetic Aharonov-Bohm flux, we find that the presence of the quantum dot has no effect on the persistent current. In contrast, the Kondo resonance interferes with the spin-dependent Aharonov-Casher effect to induce a current which, in the strong-coupling limit, is independent of the number of electrons in the ring.

Journal Article↗

PubMed2001

DNA-binding and dimerization preferences of Arabidopsis homeodomain-leucine zipper transcription factors in vitro.

Homeodomain-leucine zipper (HDZip) proteins constitute a large family of transcription factors apparently unique to plants. In this report we characterize the DNA-binding and dimerization preferences in vitro of class I HDZip proteins. Using gel-exclusion chromatography and in vitro protein binding assays we demonstrate that the HDZip class I protein ATHB5 forms a homodimeric complex in solution. Consistent with this finding we have demonstrated the sequence-specific interaction of ATHB5 with a 9 bp pseudopalindromic DNA sequence, CAATNATTG, composed of two half-sites overlapping at a central position, by use of a PCR-assisted binding-site selection assay and competitive EMSA experiments. A majority of other known members of HDZip class I interacted with similar DNA sequences, but differed in their preference for A/T versus G/C in the central position of the binding site. Selective heterodimerization in vitro was demonstrated between ATHB5 and different class I HDZip proteins. Heterodimer formation between class I HDZip proteins is of potential functional significance for the integration of information from different signalling pathways in the control of plant development.

Amino Acid Sequence↗

PubMed2001

Sugar-dependent alterations in cotyledon and leaf development in transgenic plants expressing the HDZhdip gene ATHB13.

ATHB13 is a new member of the homeodomain leucine zipper (HDZip) transcription factor family of Arabidopsis thaliana. Constitutive high-level expression of the ATHB13 cDNA in transgenic plants results in altered development of cotyledons and leaves, specifically in plants grown on media containing metabolizable sugars. Cotyledons and leaves of sugar-grown transgenic plants are more narrow and the junction between the petiole and the leaf blade less distinct, as compared to the wild type. High-level expression of ATHB13 affects cotyledon shape by inhibiting lateral expansion of epidermal cells in sugar-treated seedlings. Experiments with non-metabolizable sugars indicate that the alteration in leaf shape in the ATHB13 transgenics is mediated by sucrose sensing. ATHB13 further affects a subset of the gene expression responses of the wild-type plant to sugars. The expression of genes encoding beta-amylase and vegetative storage protein is induced to higher levels in response to sucrose in the transgenic plants as compared to the wild type. The expression of other sugar-regulated genes examined is unaffected by ATHB13. These data suggest that ATHB13 may be a component of the sucrose-signalling pathway, active close to the targets of the signal transduction.

Amino Acid Sequence↗

PubMed2001

Genetic differentiation in Eurasian populations of the postfire ascomycete Daldinia loculata.

The genetic population structure of the postfire ascomycete Daldinia loculata was studied to test for differentiation on a continental scale. Ninety-six samples of spore families, each comprising mycelia from six to 10 spores originating from single perithecia, were sampled from one Russian and six Fennoscandian forest sites. Allelic distribution was assayed for six nuclear gene loci by restriction enzyme analyses of polymerase chain reaction (PCR)-amplified gene fragments. In addition, the full sequence of the gene fragment was analysed for a subset of haploid single-ascospore isolates in a multiallelic approach. A third data set was generated by using arbitrary-primed PCR with the core sequence of the phage M13 as primer. Although there was a reduction in heterozygosity in the total population from what would have been expected at random mating, the levels of genetic differentiation among the Eurasian subpopulations of D. loculata were low. All subpopulations were found to be in Hardy-Weinberg equilibrium and gametic equilibrium was observed between all investigated nuclear gene loci. The results obtained by the different markers were consistent; we confirmed low levels of genetic differentiation among the Eurasian subpopulations of D. loculata. The differentiation did not increase with distance; the Russian subpopulation, sampled more than 7000 km from the Fennoscandian subpopulations, was only moderately differentiated from the others (FST = 0.00-0.14). In contrast, one of the Swedish populations was the most highly differentiated from the others, with FST and GST values of 0.10-0.16. The results suggest that D. loculata consists of a long-lived background Eurasian population of latent mycelia in nonburned forests, established by sexual ascospores dispersed from scattered burned forest sites. Local differentiation is probably due to founder effects of populations in areas with low fire frequency. A tentative life cycle of D. loculata is presented.

Ascomycota↗

PubMed2000

The LAMB1 gene from the clubmoss, Lycopodium annotinum, is a divergent MADS-box gene, expressed specifically in sporogenic structures.

Transcription factors encoded by the large MADS-box gene family have important developmental functions in angiosperms, the flowering plants. Mutations in certain MADS-box genes are known to cause homeotic alterations in floral organ identity, and the establishment of floral organ identity is the most well-studied developmental process in which MADS-box genes are known to function. Our interest is in the potential connection between the duplication history of this gene family and the evolutionary origin of the structures that the different MADS-box genes developmentally regulate in plants. Previous studies have demonstrated that the origin of the MADS-box genes that control floral organ identity predate the evolutionary origin of the flower itself, since gymnosperms have genes that are orthologous to angiosperm floral homeotic MADS-box genes, whereas ferns appear to lack such genes. Here we report on the isolation of a MADS-box gene from Lycopodium annotinum, which belongs to the clubmosses, the phylogenetic sister group to other vascular plants. The gene, LAMB1, in the sporophyte is expressed exclusively in the reproductive structure, the strobilus, during sporogenesis. LAMB1 is similar to other plant MADS-box genes in that it contains a MADS-box as well as a second conserved element, a K-box. However, it differs in length and in exon/intron structure in the region between the MADS- and K-box, and also in the length and structure of the C-terminal region. A phylogenetic analysis indicates that LAMB1 is not closely related to other plant-type MADS-box genes, and may represent one of the basal branches in the phylogenetic tree of plant MADS-box genes.

Amino Acid Sequence↗

PubMed2000

Two-species reaction-diffusion system with equal diffusion constants: anomalous density decay at large times

We study a two-species reaction-diffusion model where A+A-->, A+B-->, and B+B-->, with annihilation rates lambda(0), delta(0)>lambda(0), and lambda(0), respectively. The initial particle configuration is taken to be randomly mixed with mean densities n(A)(0)>n(B)(0), and with the two species A and B diffusing with the same diffusion constant. A field-theoretic renormalization group analysis suggests that, contrary to expectation, the large-time density of the minority species decays at the same rate as the majority when d</=2. Monte Carlo data support the field theory prediction in d=1, while in d=2 the logarithmically slow convergence to the large-time asymptotics makes a numerical test difficult.

Journal Article↗

PubMed1996