Reconstituted histone-DNA complexes

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Abstract
Complexes of λdv 21 plasmid DNA with calf thymus histones are reconstituted in the presence and absence of urea, and, in the absence of urea, at 4 and 37 °C. It has been concluded previously on the basis of restriction nuclease digestion of complexes reconstituted in the presence of urea that the preferential binding of individual histones to certain DNA sequences is abolished, by specific histone-histone interactions, when a mixture of the four small histones is used (Steinmetz, Streeck & Zachau 1975 Nature, Lond. 258, 447). This conclusion holds also for the other conditions tested here. An exception is the pair of arginine-rich histones (H3-H4) which appears to bind specifically when reconstituted in the absence of urea by a one step dialysis procedure and non-specifically when the salt is removed by a stepwise dialysis procedure. The subunits which are formed when the four small histones are present simultaneously are very similar to nucleosome core particles according to the fragment patterns obtained with micrococcal nuclease. Protection of 155, 285, 420, 560 etc. nucleotide pairs is observed. When histone H1 is present in addition to the four small histones a 175 nucleotide pair fragment is protected, but no 200 nucleotide pair fragment or multiples thereof were formed under our conditions.