Control of DNA polymerase gp5 chain substitution by DNA double strand annealing pressure

Abstract

DNA polymerase is essential for DNA replication and repair. As it only performs the 5′-3′ polymerization, there are two kinds of DNA replication. One of them is called strand-displacement synthesis: DNA polymerase opens the double-strand (ds) DNA to attain the 3′-5′strand (leading strand) and copy this template in a continuous way, and the other is extension synthesis: DNA polymerase copies the newly separated 5′-3′ strand (lagging strand) in a discontinuous manner. The replication complex of T7 phage is an optimal model to investigate the mechanism of replication because it is only constituted by 4 terms of protein which are DNA helicase gp4, DNA polymerase gp5 with co-factor thioredoxin (Trx), and single-strand (ss) DNA-binding protein gp2.5. The replication complex of T7 encounters both strand-displacement synthesis and extension synthesis. Previous researches reported that gp5 can have rapid extension synthesis but lacks the ability to attain strand-displacement synthesis. It also reported that gp4 translocates on ssDNA at a rapid speed but unwinds dsDNA at a very low speed. However, gp5 and gp4 together can attain rapid and processive strand-displacement synthesis. Although extensively studied, this mechanism remains unclear. Here in this work, the dynamic of strand-displacement synthesis by gp5 is investigated with single-molecule Förster (fluorescence) resonance energy transfer (smFRET). It is found that gp5, without the help of external tension, can open dsDNA but only attain strand-displacement synthesis about 4 base pairs (bp), because its exonuclease activity excises the nascent nucleotides. Therefore gp5 repeats in the synthesis-excision cycle which results in the less production of strand-displacement synthesis. We conduct another control experiment by nano-tensioner, a high precision smFRET setup which can exert a tension on dsDNA, to change the dsDNA regression pressure on gp5. It is observed that reduced dsDNA regression pressure can increase the length of strand-displacement synthesis and reduce the length of excision which indicates that the dsDNA regression pressure can regulate the strand-displacement synthesis of gp5. The further experiment shows that after gp5 and gp4 are assembled into a replisome, it can have a processive strand-displacement synthesis and barely any excision presented. The speed of replisome is a little higher than gp5 alone but much higher than gp4 alone. Additionally, the length of strand-displacement synthesis by replisome is much longer than gp5 alone. Therefore it is indicated that the gp4 can reduce dsDNA regression pressure to enables gp5 to attain processive strand-displacement synthesis. On the other hand, the gp5 facilitates gp4 to unwind the dsDNA.