Disappearance of the negative charge in giant DNA with a folding transition

In the present study we measure the electrophoretic mobility of giant T4 DN A (166 kbp) by electrophoretic light scattering for the elongated and folde d compact states at different spermidine (trivalent cation) concentrations in 50 mM sodium maleate buffer (pH 6.0). It is found that the electrophoret ic mobility of elongated DNA in the absence of the multivalent cation is se ven times greater than that of fully folded compact DNA, where, with the in crease of the concentration of spermidine, an abrupt transition is generate d after a gradual decrease of the mobility. An analysis of the electrophore tic mobility suggests that the folded compact DNA chains almost completely lose their negative charges, by taking into account the difference of frict ion mechanism between an elongated and folded compact state. From the singl e chain observation by use of fluorescence microscopy, it is found that a p hase-segregated structure is generated at intermediate concentrations of sp ermidine. The gradual decrease of the electrophoretic mobility in the trans ition region is, thus, attributed to the formation of the segregated state, exhibiting partial electroneutralization in the folded part. Disappearance of the negative charges in the completely folded compact DNAs is discussed in relation to the mechanism of transition, in terms of a first-order phas e transition.