DnaC protein

In E. coli cells, DNA replication starts at a specific site called oriC. The oriC locus contains only 245 base pairs. Similar sequences are responsible for initiating the synthesis of plasmid and bacteriophage DNA. The oriC nucleotide sequence binds several units of the tetrameric form of the dnaA protein. This protein is named for the gene that encodes it. The dnaB and dnaC proteins then bind to the complex. As a result of binding these proteins, a portion of the helical DNA is unwound. This forces the rest of the DNA into a left-handed double helix that wraps around the proteins to give a structure... 

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. 

DnaC protein 29,000 1 Required for DnaB binding at origin... 

Figure 27-19 Hypothetical scheme for initiation of bidirectional replication in E. coli. The closed boxes R1 through R5 represent the 9-residue recognition sequences for the E. coli dnaA protein. The open boxes 1, 2, and 3 represent the three 13-residue repeats, possible sites for binding of the dnaB-dnaC protein complex. From McMacken et al.m Redrawn in simplified form.

Galletto, R., and Bujalowski, W. Kinetics of the E. coli replication factor DnaC protein-nucleotide interactions. II. Fluorescence anisotropy and transient, dynamic quenching stopped-flow studies of the reaction intermediates. Biochemistry 41, 8921—8934 (2002). 

Unlike the initiation of Okazaki fragments during elongation, initiation at oriC requires RNA polymerase (in contrast to primase it is sensitive to rifampicin, see Chap. 17), and DnaA, DnaB, DnaC, and the histone-like protein HU. The role of RNA polymerase is thought to be in bringing about transcriptional activation of oriC. This presumably facilitates the multiple molecular steps leading to successful initiation. 

The DnaC/DnaB protein complex binds to both forks of the opened loop. The helicase (DnaB) acts to open the structure further. 

Suppose that a bacterial mutant is found to replicate its DNA at a very low rate. Upon analysis, it is found to have normal levels of activity of DNA polymerases 1 and 111, DNA gyrase, and DNA ligase. It also makes normal amounts of the wild-types of dnaA, dnaB, dnaC, and SSB proteins. The sequence of the oriC region of its chromosome is found to be wild type. What defect might account for the abnormally low rate of DNA replication in this mutant Explain briefly. 

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