Thermodynamics of unstable DNA structures from the kinetics of the microgene PCR.

The microgene polymerization reaction (MPR) generates head-to-tail tandem repeats from homoduplexes (HDs). In MPR initiation, one HD putatively aligns two others in the proximity required to form a nucleation complex, thus allowing the DNA polymerase to skip the intertemplate gap and generate an initial doublet (ID) prone to repeat propagation. The current investigation refines this stage by additional thermodynamic considerations and elucidates the fundamental mechanism underlying propagation. Four different HD types were designed to extend the range of melting temperatures and to simultaneously modify the stabilities of their secondary structures. Following the propagation kinetics with these, using real-time PCR at different temperatures revealed a new stage in the MPR, amplification of an ID by an original HD, and enabled us to decipher the biphasic kinetics of the process. This amplification merges with the propagation stage if the lifetime of the staggered conformation of the ID is sufficiently long for DNA polymerase to fill in the overhangs. The observed increase with temperature of thermodynamically unfavorable conformations of singlet and doublet HDs that underlies, respectively, MPR initiation and propagation is well correlated with simulations by UNAFold.