Nucleotide Sequence Determination of Xenopus borealis and Human 18S Ribosomal DNA

McCallum, Fiona S (1985) Nucleotide Sequence Determination of Xenopus borealis and Human 18S Ribosomal DNA. PhD thesis, University of Glasgow.

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Prior to this work, the complete sequence of Xenopus laevis 18S ribosomal DNA was known (Salim and Maden, 1981). Limited sequence data on Xenopus borealis included a few hundred nucleotides covering the 5' and 3' ends of the gene (Furlong and Maden, 1983). I have completed the 18S ribosomal DNA sequence for Xenopus borealis from clone pXbr101. As a result of this work, Xenopus laevis 18S ribosomal DNA has been found to contain an extra nucleotide which was formerly undetected in Maxam-Gilbert sequencing gels. The presence of this extra nucleotide was first demonstrated in Xenopus borealis by restriction analysis, and then in both species by dideoxy chain terminator sequencing. The Xenopus borealis sequence differs at only two points from the Xenopus laevis sequence. Both of these differences occur as a result of a base substitution. Both occur in regions of the sequence which are variable in comparisons between more distantly related species. The two differences are readily accommodated in the secondary structure model for Xenopus laevis 18S ribosomal RNA (Atmadja et al., 1984). The two sites of difference between Xenopus laevis and Xenopus borealis 18S ribosomal DNA were checked in a range of clones from both species. These clones contained both amplifed and chromosomal DNA. The differences were seen to be fixed within each species. Therefore, there is no intraspecies heterogeneity in Xenopus laevis or Xenopus borealis around the two points of difference. This complements the earlier findings of Maden et al., (1982a), that Xenopus laevis 18S gene regions are homogeneous. This is in direct contrast to previous data for the transcribed spacers, which show intraspecies heterogeneity and extensive interspecies divergence. (Stewart et al., 1983, Furlong and Maden, 1983; Furlong et al., 1983). I have also sequenced the complete 18S gene from human. The human sequence differs from the Xenopus borealis sequence at 123 positions (consisting of 44 insertions and 79 base substitutions). Again, these differences are in regions of the sequence known to be phylogenetically variable. Comparison of the human 18S sequence with other mammalian data reveals fewer differences. I have examined two models of 18S ribosomal RNA secondary structure (Xenopus laevis, Atmadja et al., (1984) and rat, Chan et al., 1984). The two models differ from each other in some regions. I have attempted to show (where possible) which arrangement is likely to be more correct. I have tried therefore to indicate the structure of a model which can accommodate both species and so is hopefully more representative of a consensus model for eukaryotic 18S ribosomal RNA.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Biochemistry
Date of Award: 1985
Depositing User: Enlighten Team
Unique ID: glathesis:1985-76558
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 14:09
Last Modified: 19 Nov 2019 14:09

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