Lagging Strand Synthesis: Half the DNA, Twice the Complexity

In eukaryotic nuclear DNA replication, one strand of DNA is synthesized continuously, but the other must be made as short Okazaki fragments that are later joined. The discontinuous process is inherently more complex and the steps involving fragmented intermediates create risks for disruptions of genomic integrity. Our recent observations suggest that the cell uses specific lysine (K) acetylation to alter the enzymatic properties of DNA replication and LP-BER proteins. Adding to the knowledge that acetylation partially inhibits human FEN1, we found that it also stimulates human Dna2 nuclease/helicase activities and promotes strand displacement synthesis by human pol δ and human pol β. These observations led us to hypothesize that acetylation increases the extent of RNA-DNA primer removal during Okazaki fragment synthesis. Acetylation would also increase the patch size made by pol β during BER. The likely reason for regulation by acetylation is to fully excise the error-prone primer region made by pol α and the damaged region needing repair. Involvement of p300, a histone acetyltransferase an epigenetic regulator, suggests that there are times when it is advantageous to the cell to switch between efficient and high fidelity DNA synthesis. Differential regulation of the activities of replication/repair by p300 indicates a mechanism in which the acetylase promotes formation of longer flaps in the cell at the same time as insuring correct processing. Such intentional formation of longer flaps mediated by p300 in an active chromatin environment would increase the re-synthesis patch size, providing increased opportunity for incorrect nucleotide removal during DNA replication and damaged nucleotide removal during DNA repair. Thus we propose acetylation to be a regulatory mechanism which promotes higher fidelity of DNA transactions, ensuring genome stability. Possibly this depends on development, nutritional state, or the region of the chromatin that is being duplicated.

This work is funded by National Institutes of Health grants GM098328 (to L.B.)