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Heterogeneity in cucumber ribosomal DNA.
1Department of Botany, University College, 4, Belfield, Dublin, Ireland.
Summary
Cucumber ribosomal DNA (rDNA) repeat units show significant length and sequence heterogeneity. This variation is primarily due to differences in the external spacer region and restriction fragment length polymorphism.
Area of Science:
- Molecular Biology
- Genetics
- Plant Science
Background:
- Ribosomal DNA (rDNA) comprises repeating units encoding ribosomal RNA (rRNA).
- Understanding rDNA organization and heterogeneity is crucial for comprehending genome evolution and gene expression in plants.
Purpose of the Study:
- To characterize the heterogeneity of cucumber ribosomal DNA (rDNA) repeat units.
- To identify the sources of variation in length and sequence within cucumber rDNA.
Main Methods:
- Purification of native rDNA using actinomycin-D/CsCl gradients.
- Cloning of full-length rDNA repeat units.
- Restriction enzyme digestion and analysis (EcoR I, Xba I, BamH I).
- R-loop mapping using electron microscopy.
- Hybridization with pure 18S and 25S ribosomal RNAs (rRNAs).
Main Results:
- Cucumber rDNA repeat units are heterogeneous in both length and sequence.
- Five cloned rDNA repeat units represent all EcoR I and Xba I fragments in native DNA.
- Base modification explains the absence of a BamH I site in some native rDNA.
- Major heterogeneity stems from variations in the external spacer region length.
- Restriction fragment length polymorphism was observed in EcoR I digestion.
- The coding regions for 18S and 25S rRNA are uninterrupted and conserved.
Conclusions:
- Cucumber rDNA exhibits significant structural heterogeneity, mainly in the external spacer.
- Base modification can influence restriction site presence in rDNA.
- The conserved nature of rRNA coding regions suggests functional importance.
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