Initiation of DNA replication is associated with localized melting of duplex DNA near replication origins. Helicases are loaded onto replication forks through the melted region, induced by initiator binding, in bacteria (32). We previously reported that Mcm4/6/7 can be loaded onto a bubble-like structure and can serve as a DNA helicase at the forks (13). The ability of Mcm4/6/7 to unwind the bubble substrate but not the conventional duplex DNA (Figures 2 and 13) indicates that Mcm can be loaded through the single-stranded segment of the bubble. Furthermore, the Mcm4/6/7 complex displays marked preference for thymine-rich sequences within the single-stranded segment for helicase activation (13). The results in this report indicate that helicase action of Mcm4/6/7 on synthetic bubbles may depend on the presence of an unpaired region of sufficient length (at least 40 nt), which may permit assembly of a double-hexameric complex on the substrate DNA, similar to SV40 T-antigen protein (Figure 2A) (21). When one strand of the single-stranded segment in T-rich bubble was replaced by guanine-rich sequences, the efficiency of unwinding was significantly reduced.