OriC locus

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... 

The oriC locus consists of a unique 245-bp sequence which contains a number of elements as shown in Fig. 16-15. These include four 9-bp segments called DnaA boxes which enable the oriC sequence to recognize and bind DnaA. Three 13-bp AT-rich segments at one end of oriC allow the DNA to open up during the process of initiation and provide an entry point for DnaB and DnaC. The oriC region and its immediate vicinity also contain numerous GATC sites which can be methylated. For simplicity they are not shown in Fig. 16-15. They have a role in preventing re-initiation of replication at oriC before the appropriate time in the cell cycle. 

Figure 28.50 Origin of replication in coU. The oriC locus has a length of 24S bp. It contains a tandem array of three nearly identical 13-nucleotide sequences (green) and five binding sites (yellow] for the DnaA protein.

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