Differential promotion and suppression of Z leads to B transitions in poly[d(G‐C)] by histone subclasses, polyamino acids and polyamines.

The right‐handed (B) conformation of poly[d(G‐C)] in 7.5 mM sodium cacodylate and 25% ethylene glycol can be readily converted to the left‐handed (Z) conformation by the addition of 250 microM MnCl2 and this transition can be reversed by chelation of the Mn ions with EDTA or by addition of NaCl. This ability to obtain such reversible transitions in solvent and solute conditions which allow DNA‐protein interactions and their assessment by c.d. permitted an analysis of the effect of purified histones, polyamino acids, protamine and polyamines on these transitions. Individual core histones H3, H4, H2a and H2b or protamine stabilised the Mn‐induced Z form and prevented the transition to B DNA normally observed after chelation with EDTA or on dialysis to physiological salt concentrations. A similar suppression of Z leads to B transition was also achieved with poly‐L‐arginine (but not with poly‐L‐lysine). In contrast, histones H1 and H5 promoted the Z leads to B transition. Polyamines (spermine and spermidine) converted the B form to another right‐handed (A) form which transformed to the Z form after the addition of EDTA and this Z form was restored to the B conformation on the addition of NaCl. These results suggest that sequence‐dependent variations in the conformation of natural DNA may be modulated by interaction with histones and other basic cellular components and may provide a conformational basis for nucleosome formation and possibly for the control of gene expression.