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R B Flavell

Publications and source records attributed to R B Flavell.

At least 37 records · Page 2Linked to original sources
PubMed1990

Variation and inheritance of cytosine methylation patterns in wheat at the high molecular weight glutenin and ribosomal RNA gene loci.

Chromosome marking by cytosine methylation has been examined in two gene systems in wheat--at the loci encoding high molecular weight (HMW) glutenin subunits (seed proteins) and ribosomal RNA. Variation in cytosine methylation occurs between progeny in highly inbred lines around the HMW glutenin locus. The variation is inherited through meiosis to F1 and F2 generations but occasionally a new variant arises. Specific cytosine residues lose their methyl group in the seed, the organ where the genes are expressed. Within the multigene family of ribosomal RNA genes, several subsets of genes can be defined based upon the cytosine methylation patterns. High activity of a ribosomal RNA gene locus is correlated with loss of methylation at specific cytosine residues, especially in the promoter and upstream regulatory regions. A model is described in which the subset of genes selected to be used are those to which specific regulatory proteins and transcription complexes bind most favourably. Binding of such proteins inhibits cytosine methylation and so marks the subset of genes for expression in subsequent cell generations. Examples are described where new types of RNA genes are introduced via sexual crosses that result in changes to the methylation patterns of the ribosomal RNA genes. The processes determining the changes begin, it is believed, in the fertilised egg.

Cytosine↗
PubMed1989

Conformational differences between two wheat (Triticum aestivum) 'high-molecular-weight' glutenin subunits are due to a short region containing six amino acid differences.

'High-molecular-weight' (HMW, high-Mr) glutenin subunits are protein constituents of wheat (Triticum aestivum) seeds and are responsible in part for the viscoelasticity of the dough used to make bread. Two subunits, numbered 10 and 12, are the products of allelic genes. Their amino acid sequences have been derived from the nucleic acid sequences of the respective genes. Subunit 10 has fewer amino acids than subunit 12, but migrates more slowly on SDS/PAGE (polyacrylamide-gel electrophoresis). This anomaly is due to between one and six of the amino acid differences between the subunits, localized towards the C-terminal end of the proteins. This has been established by making chimaeric genes between the genes for subunits 10 and 12, transcribing and translating them in vitro and analysing the products by SDS/PAGE. The postulated conformational differences between subunits 10 and 12 are discussed in relation to current hypotheses for the structure of HMW glutenin subunits.

Amino Acid Sequence↗
PubMed1989

A homozygous S genotype of Brassica oleracea expresses two S-like genes.

The sporophytic self-incompatibility system of Brassica species is controlled by a single locus, S. Recognition of self between pollen and stigma is probably mediated by S locus-specific glycoproteins (SLSGs). We describe the isolation, from an S29 homozygote of Brassica oleracea, of two different cDNA clones for transcripts which are equally abundant in stigmas competent for self-incompatibility and each of which is homologous to previously reported SLSG sequences. Extensive DNA sequence divergence between the two clones precludes their cross-hybridisation and each acts as a gene-specific probe. All S genotypes appear to have a single copy of each gene but there are significantly different levels of polymorphism associated with each. The clear structural homology between the two indicates a gene duplication involving the S locus and, perhaps, related to the evolution of self-incompatibility.

Amino Acid Sequence↗
PubMed1989

Tissue-specific expression of a wheat high molecular weight glutenin gene in transgenic tobacco.

The expression of a wheat genomic clone containing the entire coding sequence of the high molecular weight glutenin subunit 12 gene flanked by 2.6 kilobases of 5' and 1.5 kilobases of 3' sequences has been studied after introduction into tobacco. Seeds of different tobacco plants containing the full-length wheat genomic clone accumulated different amounts of intact high molecular weight glutenin subunit mRNA and of a polypeptide displaying the solubility, molecular weight, and antigenic properties of the high molecular weight glutenin subunit 12. The wheat protein accumulated without obvious degradation products and constituted up to approximately 0.1% of the total tobacco endosperm protein. Restriction fragments corresponding to 2.6 kilobases, 1.4 kilobases, and 433 base pairs of high molecular weight glutenin 5' upstream sequence were fused to the coding sequence of the chloramphenicol acetyltransferase (CAT) gene in the vector polyCATter and transferred into tobacco. Chloramphenicol acetyltransferase enzyme activity was detected only in the seed endosperm tissue of the transformed plants. It was detected in tobacco seeds 8 days after anthesis and persisted until seed maturity. It is concluded that 433 base pairs of high molecular weight glutenin upstream sequence are sufficient to confer endosperm-specific expression of this monocot gene in the dicot tobacco.

Chloramphenicol O-Acetyltransferase↗
PubMed1989

Mapping of ribosomal RNA transcripts in wheat.

Ribosomal RNA transcripts in wheat have been studied by RNA gel blotting and their termini determined from electrophoretic analysis of S1 nuclease-resistant RNA/DNA hybrids and also of hybrids created by primer extension. A major putative transcription initiation site has been localized 1132 base pairs upstream from the 5' end of the 18S RNA sequence. A major putative processing site occurs 640 base pairs from the 5' end of the 18S RNA. Transcripts extending at least 750 base pairs beyond the 3' end of the 25S rRNA sequence into the array of intergenic repeats are present, as are transcripts covering the principal initiation sequence. This suggests that some transcripts extend through the intergenic DNA from one repeat unit into the next.

Base Sequence↗
PubMed1988

Regulation of cytosine methylation in ribosomal DNA and nucleolus organizer expression in wheat.

Cytosine methylation has been studied in wheat rRNA genes at nucleolar organizers displaying different activities. The methylation pattern within a specific multigene locus is influenced by the number and type of rRNA genes in other rDNA loci in the cell. One CCGG site 164 base-pairs upstream from the start of transcription is preferentially unmethylated in some genes. Dominant, very active loci have a higher proportion of rRNA genes with unmethylated cytosine residues in comparison with recessive and inactive loci. It is concluded that cytosine methylation in rDNA is regulated and that the methylation pattern correlates with the transcription potential of an rRNA gene.

Binding Sites↗
PubMed1988

DNase I sensitivity of ribosomal RNA genes in chromatin and nucleolar dominance in wheat.

Ribosomal RNA genes at different nucleolar organizer (NOR) loci in hexaploid wheat are expressed at different levels. The degree of expression of a particular organizer depends on the genetic background, especially on the presence of other NOR loci. For example, when chromosome 1U of Aegilops umbellulata is introduced into the hexaploid wheat cultivar "Chinese Spring" the A. umbellulata NOR accounts for most of the nucleolar activity and seems to suppress the activity of the wheat NOR loci. Even in wild-type "Chinese Spring", the NOR on chromosome 1B is partially dominant to that on chromosome 6B, since the 1B locus is more active in spite of having fewer genes. We have previously shown that these and other examples of nucleolar dominance in wheat are associated with undermethylation of cytosine residues in certain regions of the dominant rDNA. Here, we show that rRNA genes at dominant loci are organized in a chromatin conformation that renders them more sensitive to DNase I digestion than other rRNA genes. In addition, we have mapped several DNase I-hypersensitive sites in the intergenic spacer region of the rDNA repeating unit. Some of these sites are located near the initiation region for the 45 S rRNA precursor, while others are associated with a series of short direct repeats 5' to the 45 S rRNA initiation site. The results are discussed in terms of a model in which repeated sequences in the wheat intergenic DNA are presumed to function as upstream promoters and transcriptional enhancers similar to those in Xenopus.

Binding Sites↗
PubMed1988

Structure and evolution of the intergenic region in a ribosomal DNA repeat unit of wheat.

The complete nucleotide sequence of the intergenic region between the 25 S and 18 S wheat ribosomal RNA genes has been determined from a 4.6 kb EcoRI-BamHI fragment (1 kb = 10(3) bases or base-pairs) subcloned from the plasmid pTa71. Within this subclone the intergenic DNA is flanked by the 3' end of the 25 S and the 5' end of the 18 S ribosomal RNA sequences. Four repeat families are present within the intergenic region. The major repeat family A, consists of 12 direct repeat units of 135 or 136 base-pairs (bp) flanked by diverged truncated copies. Within each A repeat a subrepeat structure has been revealed. Family B, which is localized to the 5' side of the A repeats, contains three repeat units, one of 152 bp, the second of 150 bp and a truncated unit of 107 bp. Family C, which is localized in the transcribed rRNA precursor, consists of two direct repeat units of 172 and 174 bp and possesses some short subrepeat motifs. The C repeats may have evolved by and diverged from one another by the insertion of short transposable sequences. Family D consists of two direct repeat units of 30 bp located 5' to the start of transcription. Statistical analysis of repeat family A showed that there is a significant association between the similarity of any two repeat units and their distance apart in the array. The near identity of members of the A family is maintained presumably by processes such as unequal crossing over and gene conversion, but the members at each end of the array show more divergence. Sequence motifs in the A and C repeat families and in other regions including the 5' end of 18 S RNA are related, implying that much of the intergenic DNA may have evolved from a few short ancestral sequences. The B and D repeats or their equivalent are not found in a maize ribosomal DNA repeat unit. The DNA in the external transcribed spacer DNA 5' to the 18 S RNA sequence is longer in wheat than in maize. This is due principally to two duplications and insertion of a sequence with dyad symmetry in the wheat gene.

Base Sequence↗
PubMed1987

Identification of a transposon-like insertion in a Glu-1 allele of wheat.

The Glu-1 locus, present on the long arms of the group 1 chromosomes of wheat, codes for a group of storage protein polypeptides termed high molecular weight (HMW) subunits of glutenin. Hexaploid wheat varieties carry a 'silent' Glu-1y allele on chromosome 1A, no polypeptide being attributable to this locus. When two such alleles from different varieties were compared, one was found to contain an 8 kb insertion of DNA, termed Wis-2, interrupting the coding sequence. The insertion site is flanked by a 5 bp duplication. The two ends of Wis-2 contain similar sequences over 500 bp long and its termini contain almost the same short sequences but in opposite orientation. These terminal sequences are related to those of several 'retroposon'-type transposable elements found in other organisms.

Alleles↗
PubMed1986

Repetitive DNA and chromosome evolution in plants.

Most higher plant genomes contain a high proportion of repeated sequences. Thus repetitive DNA is a major contributor to plant chromosome structure. The variation in total DNA content between species is due mostly to variation in repeated DNA content. Some repeats of the same family are arranged in tandem arrays, at the sites of heterochromatin. Examples from the Secale genus are described. Arrays of the same sequence are often present at many chromosomal sites. Heterochromatin often contains arrays of several unrelated sequences. The evolution of such arrays in populations is discussed. Other repeats are dispersed at many locations in the chromosomes. Many are likely to be or have evolved from transposable elements. The structures of some plant transposable elements, in particular the sequences of the terminal inverted repeats, are described. Some elements in soybean, antirrhinum and maize have the same inverted terminal repeat sequences. Other elements of maize and wheat share terminal homology with elements from yeast, Drosophila, man and mouse. The evolution of transposable elements in plant populations is discussed. The amplification, deletion and transposition of different repeated DNA sequences and the spread of the mutations in populations produces a turnover of repetitive DNA during evolution. This turnover process and the molecular mechanisms involved are discussed and shown to be responsible for divergence of chromosome structure between species. Turnover of repeated genes also occurs. The molecular processes affecting repeats imply that the older a repetitive DNA family the more likely it is to exist in different forms and in many locations within a species. Examples to support this hypothesis are provided from the Secale genus.

Animals↗
PubMed1981

Molecular cloning and analysis of cDNA sequences derived from poly A+ RNA from barley endosperm: identification of B hordein related clones.

A collection of over 130 cDNA clones has been constructed in the bacterial plasmids pPH207 and pBR322 using as template the poly A+ RNA from membrane-bound polysomes of barley endosperm (cv. Sundance). Fifty four B hordein cDNA clones have been identified by cross-hybridization analysis and in vitro translation of plasmid-selected mRNAs. Hybridization of 11 of the B hordein cDNA clones to Northern blots of size-fractionated RNA indicated that the B hordein mRNA is ca. 1300 nucleotides long. One cDNA clone, pHvE-c16, has been partially sequenced and shown by comparison with C-terminal and other peptide sequences to be related to B1 hordein polypeptides. The results obtained from the analysis of the B hordein cDNA clones support the idea that the Hor 2 locus, which specifies the B hordeins, is complex and codes for a family of related mRNA species.

Base Sequence↗
PubMed1980

Variations in mitochondrial DNA organisation between normal and male-sterile cytoplasms of maize.

Mitochondrial DNA from male-sterile lines of maize carrying S cytoplasm contains two small DNA species which are absent from N (fertile) and other male-sterile cytoplasms. Portions of these species have been purified and amplified by constructing recombinant plasmids in vitro. Probes made with these plasmids have been used to demonstrate; i) a homologous region in the N mitochondrial genome, which may indicate the origin of the S specific DNA species. ii) two other DNA species present in low amounts in S cytoplasm only. iii) the absence of strong homology to the S specific DNA species in mitochondria from C and T male-sterile cytoplasms.

Cytoplasm↗
PubMed1977

A first view of the meiotic process.

In this introductory paper we have highlighted some aspects of the meiotic process which seem important to us and about which some especially interesting features have been discovered. These include the switch from mitosis to meiosis, premeiotic DNA synthesis, association of the chromosomes with the synaptonemal complex, the nature of chromosome homology in relation to chromosome pairing, the process of chromosome pairing, the regulation of meiosis as a developmental process and the process of recombination. We have indulged in speculation in the hope that it will stimulate additional discussion and research into these crucial meiotic cell divisions which link the generations in higher organisms.

Animals↗
PubMed1977

Nucleotide sequence organisation in the rye genome.

Nucleotide sequence organisation in the genome of rye, Secale cereale, has been studied using renaturation kinetics and S1 nuclease digestion of the renatured products. Approximately 25 to 30% of the genome consists of very slowly renaturing, possibly single copy sequences. Most, if not all, of these sequences are interspersed between repeated sequences and are between 400 and 3500 nucleotide pairs long. Approximately 70 to 75% of the genome consists of repeated sequences. The more highly diverged repeated sequences (30% of the genome) which fail to renature under highly stringent renaturation conditions range from 500 to more than 5000 base pairs long and are interspersed in the chromosomes with less diverged repeated sequences (40% of the genome) which are usually less than 800 nucleotide pairs long. Four percent of the genome consists of very rapidly reannealing sequences which may be inverted duplications. These sequences may be in clusters distributed through at least 30% of the genome.

Base Sequence↗