DnaB helicase

FIGURE 25-12 Model for initiation of replication at the E. coli origin, oriC. (D About 20 DnaA protein molecules, each with a bound ATP, bind at the four 9 bp repeats. The DNA is wrapped around this complex. The three A=T-rich 13 bp repeats are denatured sequentially. (3) Hexamers of the DnaB protein bind to each strand, with the aid of DnaC protein. The DnaB helicase activity further unwinds the DNA in preparation for priming and DNA synthesis. 

Prokaryotic origins are initiated by DnaA, which loads in the DnaB helicase. Trans-acting Factors... 

After DnaA and DnaB binding, the DnaB helicase separates the duplex DNA strands at two replication forks. The binding of SSB to newly formed ssDNA prevents reassociation of the single strands. As the replication forks advance, the DnaA-DNA region unwraps. Subsequently DnaA is displaced. 

Replication begins when DnaA monomers bind to oriC forming a nucleosome-like structure. This requires ATP and HU. Localized melting permits the binding of the DnaB/DnaC complex. Dissociation of DnaC allows the DnaB helicase to unwind the helix in preparation for DNA replication. The DnaA complex dissociates, SSB keeps the single strands apart and the replication fork is now ready for the assembly of the replisome. 

Figure833. Phiotescence titzitioa of e-ADP, at a oonsteit conceittn tion of nucleotide, with the DnaB helicase in buffer contaun (fifferenl concentiahonsof ecaylainide. Revised and leprintcd from Ref. 61,0>f -righto 991, with peimissiMi fiiHnBaevier Science.

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