Elongation of repetitive DNA by DNA polymerase from a hyperthermophilic bacterium Thermos thermophilus

Short repetitive DNA sequences are believed to be one of the primordial gen etic elements that served as a source of complex large DNA found in the gen ome of modern organisms. However, the mechanism of its expansion (increase in repeat number) during the course of evolution is unclear. We demonstrate that the DNA polymerase of the hyperthermophilic bacterium Thermus thermop hilus can elongate oligoDNA with several tandem repeats to very long DNA in vitro. For instance, 48mer repetitive oligoDNA (TACATGTA)(6), which has 25 % GC content and a palindromic sequence, can be elongated up to similar to 10 000 bases by DNA polymerase at 74 degreesC without template DNA, OligoDN A having a different GC content or a quasipalindromic sequence can also be elongated, but less efficiently, A spectroscopic thermal melting experiment with the oligoDNA showed that its hairpin-coil transition temperature was very close to the elongation reaction temperature (74 degreesC), but was mu ch higher than the temperature at which duplex oligoDNA can exist stably. T aken together, we conclude that repetitive oligoDNA with a palindromic or q uasi-palindromic sequence is elongated extensively by a hyperthermophilic D NA polymerase through hairpin-coil transitions. We propose that such an elo ngation mechanism might have been a driving force to expand primordial shor t DNA.