Deletion analysis of a DNA sequence that positions itself precisely on the nucleosome core

Exonuclease III digests DNA sequentially from the 3' end. This enzyme is used to analyse the location of nucleosomes on DNA fragments containing a particular 145 base-pair (bp) sequence. When one of these fragments is assembled into chromatin and digested with exonuclease, a strong and persistent pause in digestion is detected at a single location. That this pause is due to the enzyme encountering a nucleosome is suggested, firstly, by its absence from digests of free DNA and, secondly, by the detection of a corresponding pause on the other strand. The two pauses, 146 bp apart, specify the location of a single precisely positioned nucleosome on the DNA fragment. This position corresponds exactly to one of two possible positions of the 145 bp sequence identified previously. A fragment containing only about 80 bp of the original 145 bp continues to position itself in the nucleosome like the parent sequence. Therefore, some of the sequence can be replaced with different DNA without affecting nucleosome positioning. Further exonuclease III analysis of an extensive set of deletions demonstrates that a central region of about 40 bp is essential for positioning the 145 bp sequence. When deletions advance into this region from either side, only a very small proportion of the DNA remains in the original position on the nucleosome. Therefore, the two short lengths of DNA at the edges of the region must each contain all or part of an essential nucleosome-positioning signal. These two critical sequences are symmetrically located across the nucleosome dyad and interact with the same region of histone H3. The sequence TGC occurs at the same place in both sequences; otherwise they are dissimilar.