DETERMINATION OF THE SIZE OF RAT RIBOSOMAL DEOXYRIBONUCLEIC-ACID REPEATING UNITS BY ELECTRON-MICROSCOPY

The employment of a novel method of affinity chromatography, which makes use of antibodies that specifically bind DNA/RNA hybrids, has made it possible to enrich for rat rDNA molecules which contain R loops formed with the 18S and 28S rRNAs. An approximately 150-fold enrichment of the rat rRNA coding sequences was obtained by this affinity chromatography procedure. This degree of enrichment made it possible to visualize these R loop containing molecules in the electron microscope and, thus, to obtain a map of the transcribed and spacer regions of rat rDNA. Eleven of the molecules that were observed contained either 3 or 4 R loops, or else 2 R loops separated by a long spacer. Thus, these molecules provided direct information in regard to the length of rat rDNA repeating units. The mean length of the repeating units was 37.2 kbp with a standard deviation of 1.3 kbp. Within the errors of the measurements, these could all represent repeating units of exactly the same length, although a certain degree of length heterogeneity, possibly up to 4 or 5 kbp, cannot be ruled out by the data. If significantly longer or shorter rDNA repeating units exist in the rat genome, they are probably much less common than the 37.2-kbp unit. These electron microscopic measurements provide the most definitive data yet available on the size of the repeating units of mammalian rRNA genes. © 1979, American Chemical Society. All rights reserved.