Nucleic Acids Research

The sequence specificity of micrococcal nuclease complicates its use in experiments addressed to the still controversial issue of nucleosome phasing. In the case of a-satellite DNA containing chromatin from African green monkey (AGM) cells cleavage by micrococcal nuclease in the nucleus was reported to occur predominantly at only one location around position 126 of the satellite repeat unit (Musich et al. (1982) Proc. Natl. Acad. Sci. USA 79, 118-122). DNA control experiments conducted in the same study indicated the presence of many preferential cleavage sites for micrococcal nuclease on the 172 bp long a-satellite repeat unit. This difference was taken as evidence for a direct and simple phase relationship between the a-satellite DNA sequence and the position of the nucleosomes on the DNA. We have quantitatively analyzed the digestion products of the protein-free satellite monomer with micrococcal nuclease and found that 50 % of all cuts occur at positions 123 and 132, 5 % at position 79, and to a level of 1-3 % at about 20 other positions. We also digested high molecular weight a-satellite DNA from AGM nuclei with micrococcal nuclease. Again cleavage occurred mostly at positions 123 and 132 of the satellite repeat unit. Thus digestion of free DNA yields results very similar to those reported by Musich et al. for the digestion of chromatin. Therefore no conclusions on a possible phase relationship can be drawn from the chromatin digestion experiments.