Journal of Molecular Biology
Volume 94, Issue 3, 25 May 1975, Pages 503-510, IN27-IN34, 511-517
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Changes in size and secondary structure of the ribosomal transcription unit during vertebrate evolution

https://doi.org/10.1016/0022-2836(75)90217-XGet rights and content

Abstract

Ribosomal RNA and precursor ribosomal RNA from at least one representative of each vertebrate class have been analyzed by electron microscopic secondary structure mapping. Reproducible patterns of hairpin loops were found in both 28 S ribosomal and precursor ribosomal RNA, whereas almost all the 18 S ribosomal RNA molecules lack secondary structure under the spreading conditions used. The precursor ribosomal RNA of all species analyzed have a common design. The 28 S ribosomal RNA is located at or near the presumed 5′-end and is separated from the 18 S ribosomal RNA region by the internal spacer region. In addition there is an external spacer region at the 3′-end of all precursor ribosomal RNA molecules. Changes in the length of these spacer regions are mainly responsible for the increase in size of the precursor ribosomal RNA during vertebrate evolution. In cold blooded vertebrates the precursor contains two short spacer regions; in birds the precursor bears a long internal and a short external spacer region, and in mammals it has two long spacer regions. The molecular weights, as determined from the electron micrographs, are 2·6 to 2·8 × 106 for the precursor ribosomal RNA of cold blooded vertebrates, 3·7 to 3·9 × 106 for the precursor of birds, and 4·2 to 4·7 × 106 for the mammalian precursor. Ribosomal RNA and precursor ribosomal RNA of mammals have a higher proportion of secondary structure loops when compared to lower vertebrates. This observation was confirmed by digesting ribosomal RNAs and precursor ribosomal RNAs with single-strandspecific S1 nuclease in aqueous solution. Analysis of the double-stranded, S1-resistant fragments indicates that there is a direct relationship between the hairpin loops seen in the electron microscope and secondary structure in aqueous solution.

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    This work has been supported by the Swiss National Science Foundation grant no. 3.872.72.

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