Abstract
The arrangement of the 26S RNA and 18S RNA sequences of the ribosomal DNA (rDNA) from the sea urchin Lytechinus variegatus was investigated by an electron microscopic analysis of R-loops formed between the ribosomal RNA genes and the mature ribosomal RNAs. Ninety-eight percent of observed molecules contained R-loops clearly seen as a three-stranded complex. The size of DNA complementary to mature cytoplasmic 18S and 26S ribosomal RNA (rRNA) was calculated by measuring the double-strand (ds) and single-strand (ss) part of the R-loops separately. The values for the 18S R-loop are 1.75±0.24 kb1 (ss) and 1.56±0.23 kb (ds). The 26S R-loop is 3.34±0.39 kb (ss) and 3.33±0.33 kb (ds). These measurements agree fairly well with the rRNA sizes measured on denaturing sucrose density gradients: 3.23±0.22 kb for the 26S and 1.93±0.10 kb for 18S. The short spacer between the 18S and 26S R-loops is 1.03±0.24 kb and the longer spacer is 5.36±0.53 kb. In long molecules a repeating pattern was observed. The average length of an rDNA repeat unit is 11.33±0.64 kb when computed using double-strand R-loop measurements and 11.50±0.72 when computed using R-loop single-strand lengths.
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Abbreviations
- kb:
-
kilobases, 1000 bases of RNA or single-strand DNA, and kilobase pairs, 1000 base pairs of duplex DNA or DNA/RNA hybrid
- EDTA:
-
ethylenediaminetetraacetate
- SSC:
-
0.15 M NaCl, 0.015 M sodium citrate
- PIPES:
-
piperazine-N,N′-bis (2-ethanesulfonic acid)-Na1.4
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Wilson, F.E., Blin, N. & Stafford, D.W. Arrangement of the rRNA sequences in the rDNA of Lytechinus variegatus . Chromosoma 65, 373–381 (1978). https://doi.org/10.1007/BF00286416
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DOI: https://doi.org/10.1007/BF00286416