Microheterogeneity in the transcribed spacers of ribosomal DNA from Xenopus laevis

Stewart, Monica A (1983) Microheterogeneity in the transcribed spacers of ribosomal DNA from Xenopus laevis. PhD thesis, University of Glasgow.

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I have examined the transcribed spacers of Xenopus laevis ribosomal DNA, by direct DNA sequence analysis, for sites of sequence variation. Substantial microheterogeneity was identified in all three transcribed spacers. The ribo-somal DNA sources examined included cloned and uncloned amplified ribosomal DNA and cloned chromosomal ribosomal DNA. In addition, amplified rDNA clones constructed from ribosomal DNA isolated and purified from a single Xenopus laeris were studied. The regions covered in DNA from these sources covered the 3' end of the external transcribed spacer and the whole of both internal transcribed spacers an area comprising approximately 1,100 nucleotides of the ribosomal DNA unit structure. Some twenty sites at which variants occur have been identified. These variants include single base changes and length variants of one to several nucleotides. In addition, a site in the first internal transcribed spacer has been shown to vary by the presence or absence of a block of twenty nucleotides The identification of sequence variants of the same type in both cloned and uncloned oocyte ribosomal DNA confirms that heterogeneities in cloned ribosomal DNA are not artefacts introduced during cloning. Comparison of the sequence data for amplified and chromosomal ribosomal DNA revealed very good correspondence between the two sources of DNA. This indicates that chromosomal ribosomal DNA serves as the primary reservoir of variant sequences. Sequence variation at the level of single nucleolar organisers was inferred in oocyte ribosomal DNA isolated from a single frog. This confirms that variants occur on physically-linked repeats within individual chromosomes. The findings of the transcribed spacers contrast strikingly with those for the ribosomal RNA coding sequences in which no sites of sequence variation were found in an extensive survey of the 18S coding region (Maden et al. 1982b). In addition, no variants were found in the 5.8S coding region in the analysis of cloned amplified and chromosomal ribosomal DNA and in a partial analysis of uncloned amplified ribosomal DNA. A single site of variation was identified near the 5' region of the 28s gene in one amplified and one chromosomal clone. The results of this study provide novel data relevant to the evolution of ribosomal DNA, especially the transcribed spacer regions, and leads to speculation about the function of these spacer regions in ribosome formation.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Ted Maden
Keywords: Biochemistry
Date of Award: 1983
Depositing User: Enlighten Team
Unique ID: glathesis:1983-73359
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2019 08:56
Last Modified: 14 Jun 2019 08:56
URI: https://theses.gla.ac.uk/id/eprint/73359

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