Length dependence in reassociation kinetics of radioactive tracer DNA

Abstract

The reassociation kinetics were measured for radioactive Escherichia coli DNA (tracers) of various average single-strand lengths reassociated alone and in the presence of excess unlabeled DNA (driver) of 2 different average lengths. Hydroxylapatite binding was used to follow the reaction time course. The length dependence of the rate constant determined in the tracer self-reassociation reactions is in agreement with the square-root dependence previously determined using optical methods to follow the time course. For the driver-tracer reactions, where the radioactive DNA reassociates largely with DNA of a different average length, the dependence of the rate constant upon average tracer length is increased and approaches an .hivin.L to the 1st power dependence. In 0.18 M Na+, the variation of the rate constant for tracer reassociation with the lengths of the reassociating strands fitted the simple equation k = (0.0077) .cntdot. (.hivin.LT)(1/.hivin.LT0.55 + 1/.hivin.LD0.55), where k is the observed rate constant in L mol-1 s-1 and .hivin.LT and .hivin.LD are the weight average tracer and driver lengths, respectively, in nucleotides. The rate of nucleation of 2 free strands is probably proportional to the sum of the reciprocals of the hydrodynamic radii of the 2 strands.