Biomedical subjects

G H Jones

Publications and source records attributed to G H Jones.

At least 19 recordsLinked to original sources

PubMed2006

Enceladus' varying imprint on the magnetosphere of Saturn.

The bombardment of Saturn's moon Enceladus by >20-kiloelectron volt magnetospheric particles causes particle flux depletions in regions magnetically connected to its orbit. Irrespective of magnetospheric activity, proton depletions are persistent, whereas electron depletions are quickly erased by magnetospheric processes. Observations of these signatures by Cassini's Magnetospheric Imaging Instrument allow remote monitoring of Enceladus' gas and dust environments. This reveals substantial outgassing variability at the moon and suggests increased dust concentrations at its Lagrange points. The characteristics of the particle depletions additionally provide key radial diffusion coefficients for energetic electrons and an independent measure of the inner magnetosphere's rotation velocity.

Atmosphere↗

PubMed2006

Control of meiotic recombination in Arabidopsis: role of the MutL and MutS homologues.

Immunocytochemistry reveals that the Arabidopsis mismatch repair proteins AtMSH4, AtMLH3 and AtMLH1 are expressed during prophase I of meiosis. Expression of AtMSH4 precedes AtMLH3 and AtMLH1 which co-localize as foci during pachytene. Co-localization between AtMSH4 and AtMLH3 occurs, but appears transient. AtMLH3 foci are not detected in an Atmsh4 mutant. However, localization of AtMSH4 is unaffected in Atmlh3, suggesting that recombination may proceed to dHj (double Holliday junction) formation. Mean chiasma frequency in Atmsh4 is reduced to 1.55 compared with 9.86 in wild-type. In contrast with wild-type, the distribution of residual crossovers in Atmsh4 closely fits a Poisson distribution. This is consistent with a two-pathway model for meiotic crossing-over whereby most crossovers occur via an AtMSH4-dependent pathway that is subject to interference, with the remaining crossovers arising via an interference-independent pathway. Loss of AtMLH3 results in an approx. 60% reduction in crossovers. Results suggest that dHj resolution can occur, but in contrast with wild-type where most or all dHjs are directed to form crossovers, the outcome is biased in favour of a non-crossover outcome. The results are compatible with a model whereby the MutL complex maintains or imposes a dHj conformation that ensures crossover formation.

Arabidopsis↗

PubMed2005

Deep Impact: observations from a worldwide Earth-based campaign.

On 4 July 2005, many observatories around the world and in space observed the collision of Deep Impact with comet 9P/Tempel 1 or its aftermath. This was an unprecedented coordinated observational campaign. These data show that (i) there was new material after impact that was compositionally different from that seen before impact; (ii) the ratio of dust mass to gas mass in the ejecta was much larger than before impact; (iii) the new activity did not last more than a few days, and by 9 July the comet's behavior was indistinguishable from its pre-impact behavior; and (iv) there were interesting transient phenomena that may be correlated with cratering physics.

Cosmic Dust↗

PubMed2005

Physical organization of the major duplication on Brassica oleracea chromosome O6 revealed through fluorescence in situ hybridization with Arabidopsis and Brassica BAC probes.

The close relationship between Brassica oleracea and Arabidopsis thaliana has been used to explore the genetic and physical collinearity of the two species, focusing on an inverted segmental chromosome duplication within linkage group O6 of B. oleracea. Genetic evidence suggests that these segments share a common origin with a region of Arabidopsis chromosome 1. Brassica oleracea and Arabidopsis bacterial artificial chromosome probes have been used for fluorescence in situ hybridization analysis of B. oleracea pachytene chromosomes to further characterize the inverted duplication. This has been highly effective in increasing the local resolution of the cytogenetic map. We have shown that the physical order of corresponding genetic markers is highly conserved between the duplicated regions in B. oleracea and the physical lengths of the regions at pachytene are similar, while the genetic distances are considerably different. The physical marker order is also well conserved between Arabidopsis and B. oleracea, with only one short inversion identified. Furthermore, the relative physical distances between the markers in one segment of B. oleracea and Arabidopsis have stayed approximately the same. The efficacy of using fluorescence in situ hybridization, together with other forms of physical and genetic mapping, for elucidating such issues relating to synteny is discussed.

Arabidopsis↗

PubMed2005

A strategy to investigate the plant meiotic proteome.

The analysis of meiosis in higher plants has benefited considerably in recent years from the completion of the genome sequence of the model plant Arabidopsis thaliana and the development of cytological techniques for this species. A combination of forward and reverse genetics has provided important routes toward the identification of meiotic genes in Arabidopsis. Nevertheless identification of certain meiotic genes remains a challenge due to problems such as limited sequence conservation between species, existence of closely related gene families and in some cases functional redundancy between gene family members. Hence there is a requirement to develop new experimental approaches that can be used in conjunction with existing methods to enable a greater range of plant meiotic genes to be identified. As one potential route towards this goal we have initiated a proteomics-based approach. Unfortunately, the small size of Arabidopsis anthers makes an analysis in this species technically very difficult. Therefore we have initially focussed on Brassica oleracea which is closely related to Arabidopsis, but has the advantage of possessing significantly larger anthers. The basic strategy has been to use peptide mass-finger printing and matrix-assisted laser desorption ionization time of flight mass spectrometry to analyse proteins expressed in meiocytes during prophase I of meiosis. Initial experiments based on the analysis of proteins from staged anther tissue proved disappointing due to the low level of detection of proteins associated with meiosis. However, by extruding meiocytes in early prophase I from individual anthers prior to analysis a significant enrichment of meiotic proteins has been achieved. Analysis suggests that at least 18% of the proteins identified by this route have a putative meiotic function and that this figure could be as high as one-third of the total. Approaches to increase the enrichment of proteins involved in meiotic recombination and chromosome synapsis are also described.

Brassica↗

PubMed2003

Meiotic chromosome synapsis and recombination in Arabidopsis thaliana; an integration of cytological and molecular approaches.

Arabidopsis has emerged as an important model for the analysis of meiosis in Angiosperm plants, creating an interesting and useful parallel to other model organisms. This development has been underpinned by advances in the molecular biology and genetics of Arabidopsis, especially the determination of its entire genome sequence. However, these advances alone would have been insufficient without the development of improved methods for cytological analysis and cytogenetic investigation of meiotic nuclei and chromosomes. A basic descriptive framework of meiosis in Arabidopsis has been established based on these procedures. In addition, molecular cytogenetic and immunocytological techniques have provided supplementary detailed information on some aspects of meiosis. Gene identification and characterization have proceeded in parallel with these developments based on both forward and reverse genetic procedures utilising the considerable range of Arabidopsis genetic and molecular resources, such as T-DNA and transposon tagged lines as well as the genomic DNA database, in combination with cytological analysis. A diverse range of meiotic genes have been identified and analysed by these procedures and in selected cases they have been subjected to detailed functional analysis. This review focuses on genes that are involved in the key meiotic events of chromosome synapsis and recombination.

Arabidopsis↗

PubMed2003

Partial diploidization of meiosis in autotetraploid Arabidopsis thaliana.

Meiosis was analyzed cytogenetically in autotetraploids of Arabidopsis, including both established lines and newly generated autotetraploid plants. Fluorescent in situ hybridization with 5S and 45S rDNA probes was used to identify the different chromosomes at metaphase I of meiosis. Multivalents were observed frequently in all the lines analyzed, but there were significant differences in multivalent frequency not only between the newly generated tetraploids and the established lines but also among the different established lines. The new tetraploids showed high multivalent frequencies, exceeding the theoretical 66.66% predicted by the simple random-end pairing model, in some cases significantly, thus indicating that Arabidopsis autotetraploids have more than two autonomous pairing sites per chromosome, despite their small sizes. The established lines showed fewer multivalents than the new autotetraploids did, but the extent of this reduction was strongly line and chromosome dependent. One line in particular showed a large reduction in multivalents and a concomitant increase in bivalents, while the other lines showed lesser reductions in multivalents. The reduction in multivalents was not uniformly distributed across chromosomes. The smaller chromosomes, especially chromosomes 2 and 4, showed the most marked reductions while the largest chromosome (1) showed virtually no reduction compared to the new tetraploids. It is concluded that the established autotetraploid lines have undergone a partial diploidization of meiosis, but not necessarily genetical diploidization, since their creation. Possible mechanisms for the resulting change in meiotic chromosome behavior are discussed.

Arabidopsis↗

PubMed2002

Variation in chiasma frequency among eight accessions of Arabidopsis thaliana.

Natural variation in meiotic recombination frequency in Arabidopsis thaliana has been assessed by analyzing chiasma frequency variation among a range of geographically and ecologically diverse accessions. Fifty pollen mother cells at metaphase I of meiosis were analyzed from each of eight accessions and fluorescence in situ hybridization was applied to enable identification of all 10 chromosome arms. There was no significant variation in mean chiasma frequency between plants within accessions, but there was significant variation between accessions. Further analysis confirmed this finding and identified two particular accessions, Cvi and Ler, as having chiasma frequencies significantly lower than those of the other accessions. The analysis also revealed that the pattern of chiasma distribution between arms and among chromosomes is not consistent over accessions. Further detailed analyses were conducted on each individual chromosome (1-5) in turn, revealing that chromosome 4, one of the acrocentric chromosomes, is the least variable while the other acrocentric chromosome (2) is the most variable. These findings indicate the existence of recombination regulatory elements in Arabidopsis and we conclude that it may be possible in the future to identify these elements and determine their mode of action. The practical implications of such developments are considerable.

Analysis of Variance↗

PubMed2001

Transcriptional analysis and regulation of the sigma-E gene of Streptomyces antibioticus.

We report here the mapping of the transcriptional start point and identification of the promoter for the sigE gene of Streptomyces antibioticus. Sequence analysis revealed a conserved genetic organization of five genes encompassing sigE in S. antibioticus and S. coelicolor. Upstream of sigE a number of direct repeats, while conserved in both species, are arranged differently. Gel shift analysis demonstrated binding of a component of both S. antibioticus and S. coelicolor crude protein extracts to a 30 bp sequence encompassing one repeat, the A-rich box. Deletion analysis in promoter probes showed that maximal activity of the S. antibioticus promoter depends upon the presence of the sequence surrounding the A-rich box, as well as the region further upstream carrying other direct repeats.

Bacterial Proteins↗

PubMed2001

Chiasma formation in Arabidopsis thaliana accession Wassileskija and in two meiotic mutants.

Meiotic chiasmata were analysed in metaphase I pollen mother cells (PMCs) of wild-type Arabidopsis thaliana and in two meiotic mutants. Fluorescence in situ hybridisation (FISH) with 45S rDNA and 5S rDNA as probes was used to identify the five chromosome pairs. A wild-type chiasma frequency of 9.24 per cell was found, consistent with estimated genetic recombination values. Individual bivalent chiasma frequencies varied according to chromosome size; chromosome 1 had the highest mean chiasma frequency (2.14) while the short acrocentric chromosomes had the lowest frequencies (1.54 and 1.56). FISH analysis was extended to two meiotic mutants (asy1 and dsy1) having low residual bivalent and chiasma frequencies. Mutant dsy1 gave no indication of chromosome preference for residual bivalent formation; instead it showed a general reduction in bivalent and chiasma frequencies. In asy1, the longest chromosome (1) had the lowest bivalent frequency and chiasma frequency while the short acrocentric chromosome 2 had the highest frequencies. This chromosome pair may be preferentially involved in synapsis and chiasma formation because of their association with the nucleolus. However, other factors may be operating since the other acrocentric chromosome (4), with similar size and structure to chromosome 2, did not share these chiasma properties.

Arabidopsis↗

PubMed2001

Poly(A) polymerase activity and RNA polyadenylation in Streptomyces coelicolor A3(2).

The Streptomyces coelicolor genome sequence was searched for open reading frames (ORFs) similar to Escherichia coli poly(A) polymerase I, revealing an ORF with 36% amino acid sequence identity to that protein. Mycelial extracts prepared from S. coelicolor cultures incorporated radioactive ATP into an acid-insoluble form, and some of the products of this incorporation had the properties expected of poly(A). [3H]-uridine and [3H]-adenosine were used to label the RNA in S. coelicolor cultures of different ages, and total RNA was fractionated by oligo dT cellulose chromatography. Approximately 3% of the total uridine-labelled RNA and 11% of the adenosine-labelled RNA were retained by the oligo dT cellulose columns. Enzymatic digestion of the retained RNA supported the conclusion that a significant fraction of the adenosine label was present in 3'-poly(A) chains. Measurement of poly(A) tail lengths by end labelling of total RNA and RNase digestion revealed a maximum length of approximately 18 residues. Radioactive cDNA prepared from the RNA fraction retained by oligo dT cellulose hybridized to the 16S and 23S genes from a streptomycete ribosomal RNA operon but not to the 5S gene. Reverse transcription-polymerase chain reaction (RT-PCR) revealed the presence of mRNAs in the RNA fraction retained by oligo dT cellulose.

Amino Acid Sequence↗

PubMed2001

Nucleolus-associated telomere clustering and pairing precede meiotic chromosome synapsis in Arabidopsis thaliana.

The intranuclear arrangements of centromeres and telomeres during meiotic interphase and early prophase I of meiosis in Arabidopsis thaliana were analysed by fluorescent in situ hybridisation to spread pollen mother cells and embryo-sac mother cells. Meiocyte identification, staging and progression were established by spreading and sectioning techniques, including various staining procedures and bromodeoxyuridine labeling of replicating DNA. Centromere regions of Arabidopsis are unpaired, widely dispersed and peripherally located in nuclei during meiotic interphase, and they remain unpaired and unassociated throughout leptotene. Eventually they associate pairwise during zygotene, as part of the nucleus-wide synapsis of homologous chromosomes. Telomeres, by contrast, show a persistent association with the nucleolus throughout meiotic interphase. Variation in telomere signal number indicates that telomeres undergo pairing during this interval, preceding the onset of general chromosome synapsis. During leptotene the paired telomeres lose their association with the nucleolus and become widely dispersed. As the chromosomes synapse during zygotene, the telomeres reveal a loose clustering within one hemisphere, which may represent a degenerate or relic bouquet configuration. We propose that in Arabidopsis the classical leptotene/zygotene bouquet is absent and is replaced functionally by nucleolus-associated telomere clustering.

Arabidopsis↗

PubMed2000

A duplicated fold is the structural basis for polynucleotide phosphorylase catalytic activity, processivity, and regulation.

BACKGROUND: Polynucleotide phosphorylase (PNPase) is a polyribonucleotide nucleotidyl transferase (E.C.2.7.7.8) that degrades mRNA in prokaryotes. Streptomyces antibioticus PNPase also assays as a guanosine 3'-diphosphate 5'-triphosphate (pppGpp) synthetase (E.C.2.7.6.5). It may function to coordinate changes in mRNA lifetimes with pppGpp levels during the Streptomyces lifecycle. RESULTS: The structure of S. antibioticus PNPase without bound RNA but with the phosphate analog tungstate bound at the PNPase catalytic sites was determined by X-ray crystallography and shows a trimeric multidomain protein with a central channel. The structural core has a novel duplicated architecture formed by association of two homologous domains. The tungstate derivative structure reveals the PNPase active site in the second of these core domains. Structure-based sequence analysis suggests that the pppGpp synthetase active site is located in the first core domain. CONCLUSIONS: This is the first structure of a PNPase and shows the structural basis for the trimer assembly, the arrangement of accessory RNA binding domains, and the likely catalytic residues of the PNPase active site. A possible function of the trimer channel is as a contribution to both the processivity of degradation and the regulation of PNPase action by RNA structural elements.

Amino Acid Sequence↗

PubMed2000

Integrated cytogenetic map of chromosome arm 4S of A. thaliana: structural organization of heterochromatic knob and centromere region.

We have constructed an integrated cytogenetic map of chromosome arm 4S of Arabidopsis thaliana. The map shows the detailed positions of various multicopy and unique sequences relative to euchromatin and heterochromatin segments. A quantitative analysis of the map positions at subsequent meiotic stages revealed a striking pattern of spatial and temporal variation in chromatin condensation for euchromatin and heterochromatin. For example, the centromere region consists of three domains with distinguishable structural, molecular, and functional properties. We also characterized a conspicuous heterochromatic knob of approximately 700 kb that accommodates a tandem repeat and several dispersed pericentromere-specific repeats. Moreover, our data provide evidence for an inversion event that relocated pericentromeric sequences to an interstitial position, resulting in the heterochromatic knob.

Arabidopsis↗

PubMed2000

A homologue of the yeast HOP1 gene is inactivated in the Arabidopsis meiotic mutant asy1.

Synapsis of homologous chromosomes is a key event in meiosis as it is essential for normal chromosome segregation and is implicated in the regulation of crossover frequency. We have previously reported the identification and cytological characterisation of a T-DNA-tagged asynaptic mutant of Arabidopsis thaliana. We have demonstrated that this mutant, asy1, is defective in meiosis in both males and females. Cloning and nucleotide sequencing of the ASY1 gene has revealed that it encodes a polypeptide of 596 amino acids that exhibits similarity to the HOP1 gene of Saccharomyces cerevisiae, which is known to encode a protein essential for synaptonemal complex assembly and normal synapsis. Expression studies indicate that, in common with a number of other Arabidopsis meiotic genes, ASY1 exhibits low-level expression in a range of plant tissues. Southern analysis coupled with database searching has resulted in the identification of an ASY1 homologue, ASY2. Although asy1 exhibits a strong asynaptic phenotype, a residual low level of synapsis indicates that ASY1 and ASY2 may exhibit a low degree of functional redundancy.

Arabidopsis↗

PubMed2000

Actinomycin production persists in a strain of Streptomyces antibioticus lacking phenoxazinone synthase.

Truncated fragments of the phenoxazinone synthase gene, phsA, were prepared by the PCR. The resulting fragments were cloned into conjugative plasmid pKC1132 and transferred to Streptomyces antibioticus by conjugation from Escherichia coli. Two of the resulting constructs were integrated into the S. antibioticus chromosome by homologous recombination, and each of the resulting strains, designated 3720/pJSE173 and 3720/pJSE174, contained a disrupted phsA gene. Strain 3720/pJSE173 grew poorly, and Southern blotting suggested that genetic changes other than the disruption of the phsA gene might have occurred during the construction of that strain. Strain 3720/pJSE174 sporulated well and grew normally on the medium used to prepare inocula for antibiotic production. Strain 3720/pJSE174 also grew as well as the wild-type strain on antibiotic production medium containing either 1 or 5.7 mM phosphate. Strain 3720/pJSE174 was shown to be devoid of phenoxazinone synthase (PHS) activity, and PHS protein was undetectable in this strain by Western blotting. Despite the absence of detectable PHS activity, strain 3720/pJSE174 produced slightly more actinomycin than did the wild-type parent strain in medium containing 1 or 5.7 mM phosphate. The observation that strain 3720/pJSE174, lacking detectable PHS protein or enzyme activity, retained the ability to produce actinomycin supports the conclusion that PHS is not required for actinomycin biosynthesis in S. antibioticus.

Cell Division↗

PubMed1999

The DIF1 gene of Arabidopsis is required for meiotic chromosome segregation and belongs to the REC8/RAD21 cohesin gene family.

Cohesins are a group of conserved proteins responsible for cohesion between replicated sister chromatids during mitosis and meiosis and which are implicated in double-strand break repair and meiotic recombination. We describe here the identification and characterisation of an Arabidopsis gene - DETERMINATE, INFERTILE1 (DIF1), which is a homolog of the Schizosaccharomyces pombe REC8/RAD21 cohesin genes, and is essential for meiotic chromosome segregation. Five independent alleles of the DIF1 gene were isolated by transposon mutagenesis, and the mutants show complete male and female sterility. Pollen mother cells (PMCs) of dif1 mutants show multiple meiotic defects which are represented by univalent chromosomes and chromosome fragmentation at metaphase I, and acentric fragments and chromatin bridges in meiosis I and II. Consequently, chromosome segregation is strongly affected, resulting in meiotic products of uneven size, shape and of variable ploidy. The similarities in phenotype, and the sequence homology between DIF1 and the REC8/RAD21 cohesins suggests that cohesin function is largely conserved between eukaryotes and highlights the essential role cohesins play in plant meiosis.

Alleles↗

PubMed1999

relA is required for actinomycin production in Streptomyces antibioticus.

The relA gene from Streptomyces antibioticus has been cloned and sequenced. The gene encodes a protein with an Mr of 93,653, which is 91% identical to the corresponding protein from Streptomyces coelicolor. Disruption of S. antibioticus relA produces a strain which grows significantly more slowly on actinomycin production medium than the wild type or a disruptant to which the intact relA gene was restored. Moreover, the disruptant was unable to accumulate ppGpp to the levels observed during the normal course of growth and actinomycin production in the wild type. The strain containing the disrupted relA gene did not produce actinomycin and contained significantly lower levels of the enzyme phenoxazinone synthase than the wild-type strain. Actinomycin synthetase I, a key enzyme in the actinomycin biosynthetic pathway, was undetectable in the relA disruptant. Growth of the disruptant on low-phosphate medium did not restore actinomycin production.

Amino Acid Sequence↗