CHROMATIN STRUCTURE OF THE 5S-RIBONUCLEIC ACID GENES OF XENOPUS-LAEVIS

Staphylococcal nuclease digestion has been used to investigate the structure of bulk chromatin and of that containing the genes for 5S RNA in two tissues from Xenopus laevis. In red blood cells, the nucleosome repeat length of the majority of the chromatin is 189 ± 4 bp. In nuclei prepared from liver cells, two populations of nucleosome repeat lengths were detected. The majority of the chromatin has a repeat length of 178 ± 5 bp, but, at late times of digestion with staphylococcal nuclease, a fraction of chromatin with a repeat length of 155 ± 8 bp is apparent. Oocyte-type 5S DNA comprises 0.38% of the X. laevis genome, or about 30 000 repeating units per diploid cell, but is not transcribed in somatic tissues. During nuclease digestion, the fate of these sequences has been monitored by hybridization with specific probes. By using the blotting technique of E. M. Southern ((1975) J. Mol. Biol. 98, 503-517), the nucleosome repeat length of 5S DNA is 175 ± 5 bp in both blood and liver, suggesting that these specific genes may have a constant chromatin structure, independent of the organization of bulk chromatin in the two tissues. In addition, 5S DNA is less susceptible to staphylococcal nuclease than is bulk DNA in nuclei from both cell types. Quantitative hybridization techniques demonstrate that, with increasing degrees of digestion, high molecular weight nucleosome multimers are enriched up to threefold in 5S DNA sequences. Concomitantly, the 5S DNA content of monomer DNA from these digests is reduced. Calculations indicate that the oocyte 5S DNA sequences, which in the tissues studied are transcriptionally silent, are cut by staphylococcal nuclease at 60-80% of the rate that the bulk DNA is cut. © 1979, American Chemical Society. All rights reserved.