Processive DNA helicase activity of the minichromosome maintenance proteins 4, 6, and 7 complex requires forked DMA structures

The minichromosome maintenance (Mcm) proteins 2-7 are required for both the initiation and elongation steps of chromosomal DNA replication. Previous s tudies have shown that the Mcm complex consisting of the Mcm 4, 6, and 7 pr oteins contains 3' to 5' DNA helicase activity with limited processivity (d isplacing duplex DNA regions up to 30 nt). In this report, we show that the presence of both 5' and 3' single-stranded tails in DNA helicase substrate s is essential for the processive helicase activity of the Mcm complex. The presence of both 5' and 3' tails facilitated the formation of double heter ohexameric complexes of Mcm4/6/7 on substrate DNA, which appeared to be ess ential for the processive helicase activity. The double heterohexameric com plex of Mcm4/6/7, in the presence of a single-strand DNA binding protein, i s capable of unwinding duplex DNA region of about 600 bp in length. These r esults support the hypothesis that the Mcm4/6/7 complex can function as a r eplication helicase.