Denaturation map of the ribosomal DNA of Xenopus laevis

Abstract

A denaturation map of the ribosomal DNA of Xenopus laevis has been constructed. A denaturation pattern repeats every 5.4 ± 0.4 μ within this DNA. By comparing the contour length of these repeats with the lengths of intact bacteriophage T7 DNA molecules of known molecular weight, the repeating unit in ribosomal DNA has been calculated to be 8.7 ± 0.6 × 10 6 daltons. Within each repeating unit there is a region of about 5.0 × 10 6 daltons which denatures under conditions where the remaining part of the repeat (about 3.7 × 10 6 daltons) remains essentially native. The highly denatured region coincides in molecular weight to the gene which codes for the 40 s RNA precursor molecule. The region which remains native is presumed to comprise the “spacer” sequences of ribosomal DNA which are known to have a higher G + C content than the DNA which codes for the 40 s RNA. Symmetrically placed within the presumed 40 s gene region are three DNA sequences which remain native even under conditions which have largely denatured the DNA within the spacer region. The ribosomal DNA repeats are homogeneous. The mean repeat lengths within every molecule tested do not differ by statistical test, and the average length of all repeats measured has a standard deviation only 1.6 times larger than that found in the lengths of homogeneous T7 phage DNA molecules. The distribution of native and denatured regions is similar in all repeats. Amplified and chromosomal ribosomal DNA are indistinguishable with respect to length, denaturation pattern and arrangement of repeats. These and earlier studies are consistent with the hypothesis that the ribosomal DNA chromosomal locus consists of several hundred identical or nearly identical genes in a tandem repeating arrangement.