The origin of replication in E.coli is called the oriC
This region is rich in Adenine and Thymine bases because their base stacking interactions are weaker than those of Cytosine and Guanine.
Between A and T bases there are only 2 hydrogen bonds (not a very strong interaction so requires less energy to separate) while between C and G bases there are 3 hydrogen bonds (strong interaction thus requires more energy to separate)
The oriC is a region which needs to be separated when replication occurs. It is thus a region which requires easy separation with as little energy as possible so that proteins like DnaA can bind.
