In vitro expansion of mammalian telomere repeats by DNA polymerase α-primase

Among the polymerases, DNA polymerase α-primase is involved in lagging strand DNA synthesis. A previous report indicated that DNA polymerase α-primase initiates primer RNA synthesis with purine bases on a single-stranded G-rich telomere repeat. In this study, we found that DNA polymerase α-primase precisely initiated with adenosine opposite the 3'-side thymidine in the G-rich telomere repeat 5'-(TTAGGG)(n)-3' under rATP-rich conditions. Then, DNA polymerase α-primase synthesized the nascent DNA fragments by extending the primer. It was remarkable that DNA polymerase α-primase further expanded the product DNA far beyond the length of the template DNA, as ladders of multiple hexanucleotides on polyacrylamide gel electrophoresis. Using an oligomer duplex 5'-A(GGGTTA)₅-3'/5'-(TAACCC)₅T-3' as a template-primer, we show that both the Klenow fragment of Escherichia coli DNA polymerase I and HIV reverse transcriptase could expand telomere DNA sequences as well, giving products greater than the size of the template DNA. The maximum product lengths with these polymerases were ~40-90 nt longer than the template length. Our data imply that DNA polymerases have an intrinsic activity to expand the hexanucleotide repeats of the telomere sequence by a slippage mechanism and that DNA polymerase α uses both the repeat DNA primers and the de novo RNA primers for expansion. On the other hand, a plasmid harboring a eukaryotic telomere repeat showed remarkable genetic instability in E.coli. The telomere repeats exhibited either expansions or deletions by multiple hexanucleotide repeats during culture for a number of generations, suggesting involvement of the slippage mechanism in the instability of telomeric DNA in vivo.