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R O Williams

Publications and source records attributed to R O Williams.

At least 109 records · Page 6Linked to original sources
PubMed1982

analysis of genomic rearrangements associated with two variable antigen genes of Trypanosoma brucei.

Some variable surface glycoprotein (VSG) genes of Trypanosoma brucei undergo duplication and transposition when they are expressed. We report here the cloning of cDNAs coding for two VSGs from the ILtar 1 repertoire. Analysis of the genomes of trypanosomes expressing these and other antigens shows that there is no additional copy of the sequences coding for eight VSG in expressing clones of trypanosomes, and reveals rearrangements analogous to those previously described for the gene for another VSG from this antigen repertoire. The data indicate that duplication does not accompany the expression of these VSG genes. Transposition to a specific expression site cannot be excluded, but would have to involve either a much larger segment of DNA, or movement to a region of much greater homology with the previous flanking sequences, than is observed for VSG genes that are duplicated when expressed. It is reasoned that the control of expression by coupled duplication and transposition is not sufficient to account for the selection of a single VSG gene for expression.

Animals↗
PubMed1981

Contextural genomic rearrangements of variable-antigen genes in Trypanosoma brucei.

We have described some rearrangements of a variable-antigen gene in T. brucei. We suggest that there are two copies of the ILTat 1.2 variable-antigen gene in each of a number of trypanosome clones closely related by sequential relapses. Both copies of the gene undergo rearrangements, apparently the result of insertion and deletion of lengths of DNA in a region at or beyond the 3' end of the coding sequence, giving rise to different-size restriction enzyme fragments in different clones of trypanosomes. No feature of these rearrangements can be correlated with expression of the gene. Our data differ from those of Hoeijmakers et al. (1980) in two important respects: (1) Neither copy of the gene remains in a constant genomic context in all trypanosome clones. (2) We do not see an extra copy associated with the expression of the gene. These observations do not suggest any obvious mechanism for the phenomenon of antigenic variation.

Animals↗
PubMed1980

Analyses of variable antigen gene rearrangements in Trypanosoma brucei.

For the purpose of investigating the genetic basis of antigenic variation in Trypanosoma brucei, we have analyzed the structure of the genome surrounding the gene coding for one T. brucei variable antigen (ILTat 1.2) in several T. brucei clones expressing this and other variable antigens. In each case there are two copies of the gene. We found no evidence of an extra copy associated with the expression of this gene. Differences were found between the two copies in a single clone, and between the copies in different clones. The differences could be explained by insertion and deletion of various lengths of DNA in a region beyond the C-terminal end of the gene. Differences in genomic structure were found even between clones expressing the same antigen, whether ILTat 1.2 or another. Thus, no feature of the rearrangements observed can be correlated with the expression of a particular antigen.

Animals↗
PubMed1979

Detection and characterization of DNA polymerase from Trypanosoma brucei.

The predominant DNA polymerase activity has been isolated from the parasitic flagellated protozoan, Trypanosoma brucei. Like mammalian DNA polymerase-alpha the trypanosome DNA polymerase is of large molecular weight (S, 6--8), is resistant to thermal denaturation, is sensitive to N-ethylmaleimide, and is inhibited by high ionic strength. However, specific antisera that cross-react with mammalian DNA polymerase-alpha from different species fail to cross-react with the trypanosome polymerase.

Animals↗
PubMed1978

A characterization of mRNA activites and their sequence complexities in Trypanosoma brucei: partial purification and properties of the VSSA mRNA.

Polyadenylated RNA isolated from a clone of Trypanosoma brucei was shown to direct the synthesis of a variety of polypeptides in a cell-free system. A predominant 58,000 dalton polypeptide was immunoprecipitated with antisera to the T. brucei variant specific surface antigen (VSSA). The mRNA that directed the synthesis of the VSSA was 2.0 kilobases (kb) long as measured by polyacrylamide gel electrophoresis in 98% formamide. Complementary DNA was prepared with avian myeloblastosis virus reverse transcriptase and the nucleotide sequence complexities of the total polysomal poly(A)+RNA and a gel purified VSSA mRNA were measured. 20% of the total cellular poly(A)+RNA contained abundant sequences with an apparent complexity of 9.6 kb; 42% of the purified VSSA mRNA contained abundant sequences with a complexity of 7.2 kb. Complementary DNA synthesized from gel purified VSSA mRNA was hybridized to total cellular poly(A)+RNA isolated from an unrelated T. brucei clone expressing a different variant antigen. A portion of the low complexity RNA sequence component was absent in the heterologous mRNA population but the same plateau of hybridization was achieved (93%). The abundance of some of the low complexity mRNAs appears to be T. brucei clone specific.

Animals↗
PubMed1973

Zinc requirement for DNA replication in stimulated human lymphocytes.

The requirement for Zn(++) in DNA replication by phytohemagglutinin-stimulated human lymphocytes was studied. When 6 microM o-phenanthroline, a chelator with a high affinity for Zn(++), is added to cultures of stimulated lymphocytes a nearly complete inhibition of thymidine incorporation results within a few hours. In contrast, the incorporation of uridine is only slightly reduced and the incorporation of leucine is unaffected. m-Phenanthroline, a nonchelating analogue, does not alter the rate of thymidine incorporation even when present in 10-fold greater amounts than o-phenanthroline. The inhibition of thymidine incorporation by o-phenanthroline could be entirely reversed by the addition of Zn(++) to the cultures, or could be prevented by the prior addition of either Zn(++) or Ni(++). All other divalent cations tested were incapable of reversing the o-phenanthroline inhibition of thymidine incorporation.

Blood Proteins↗
PubMedDate not supplied

Genomic rearrangements correlated with antigenic variation in Trypanosoma brucei.

The capacity of African trypanosomes to express sequentially a large repertoire of different surface antigens during an infection enables the parasite to evade the immune response of its host, and makes attempts to produce a vaccine against the disease difficult. It is evident that point mutations cannot account for antigen diversity. Variable antigens like immunoglobulins are derived from an extensive family of genes of which only one is expressed in a given cell. As somatic tic recombination is involved in the immunoglobulin gene system, this similarity prompted us to search for somatic rearrangements in trypanosome variable antigen genes. We have constructed a recombinant plasmid containing approximately half the DNA sequence coding for a Trypanosoma brucei variable antigen and hybridised the inserted sequences to various restriction enzyme digests of nuclear DNA from different trypanosome clones. Differences in the sizes of restriction tion fragments hybridising to the inserted variable antigen coding sequence show altered positions of enzyme sites relative to this sequence, indicating different arrangements of DNA sequences around this gene in different trypanosome clones.

Animals↗
PubMedDate not supplied

Are there two classes of VSG gene in Trypanosoma brucei?

Antigenic variation in the African trypanosomes involves the sequential expression of genes coding for different variant surface glycoproteins (VSGs) (reviewed in refs 1-3). When expression of some VSG genes is switched on, a newly duplicated copy of the expressed gene has been observed within the trypanosome genome, which is not found after the gene's expression is switched off again. The duplicated copy has therefore been called an expression-linked copy (ELC). The expression of the gene appears to be strictly coupled to the presence of the ELC. This has led to the hypothesis that the duplicative transposition generating the ELC may itself be responsible for the control of VSG expression. With other VSG genes, expression-linked duplication has not been observed, and expression is clearly not controlled in this way. Data are presented here which demonstrate that either of these observations may be obtained with a single VSG gene, depending on the chance selection of particular clones from antigenically switched populations. Thus, the different observations do not imply the existence of two distinct classes of VSG gene controlled by different mechanisms, but different aspects of processes common to all VSG genes.

Animals↗
PubMedDate not supplied

Combination therapy with DMARDs and biological agents in collagen-induced arthritis.

There is increasing interest in the use of combination therapy for rheumatoid arthritis and in the possibility of combining the conventional drug approach with newer biological therapies. Animal models of arthritis provide important tools for evaluating novel forms of therapy and for eludicating mechanisms of drug action. In this paper, we review the results of our own research into combination therapy in collagen-induced arthritis using biological therapies such as anti-tumor necrosis factor alpha, anti-CD4, and anti-interleukin 12 monoclonal antibodies, and small molecular weight compounds such as cyclosporin and the phosphodiesterase IV (PDE IV) inhibitor rolipram.

Animals↗