Regular arrangement of nucleosomes on 5S rRNA genes in Xenopus laevis.

The chromatin structure of the oocyte-type 5S RNA genes in Xenopus laevis was investigated. Blot hybridization analysis of DNA from micrococcal nuclease digests of erythrocyte nuclei showed that 5S DNA has the same average nucleosome repeat length, 192 +/- 4 base pairs, as two Xenopus satellite DNAs and bulk erythrocyte chromatin. The positions of nuclease-sensitive regions in the 5S DNA repeats of purified DNA and chromatin from erythrocytes were mapped by using an indirect end-labeling technique. Although most of the sites cleaved in purified DNA were also cleaved in chromatin, the patterns of intensities were strikingly different in the two cases. In 5S chromatin, three nuclease-sensitive regions were spaced approximately a nucleosome length apart, suggesting a single, regular arrangement of nucleosomes on most of the 5S DNA repeats. The observed nucleosome locations are discussed with respect to nucleotide sequences known to be important for expression of 5S RNA. Because the preferred locations appear to be reestablished in each repeating unit, despite spacer length heterogeneity, we suggest that the regular chromatin structure reflects the presence of a sequence-specific DNA-binding component on inactive 5S RNA genes.