OriC

oriC is the chromosomal origin of replication in bacteria, the cis-acting site where DNA replication is initiated. In most bacteria, replication starts at a single oriC locus on the circular chromosome and proceeds bidirectionally from that point. The oriC region is typically short and its exact sequence is species-specific, but it generally contains binding sites for the initiator protein DnaA and an AT-rich region that opens easily to start replication.

Sequence features of oriC include multiple DnaA boxes, short consensus sequences to which DnaA binds. A common core motif is 9 base pairs long with a consensus such as TTATNCACA, arranged in clusters that enable cooperative DnaA binding. The region also contains a DNA unwinding element (DUE), an AT-rich stretch that becomes single-stranded to allow loading of the replication machinery. In Escherichia coli, the oriC is about 245 base pairs in length and lies near the dnaA gene, but other bacteria show different lengths and arrangements of DnaA boxes and adjacent regulatory elements.

Initiation mechanism involves DnaA-ATP binding to oriC, promoting oligomerization and DNA bending that facilitate local unwinding of the DUE. This open complex permits loading of the DnaB helicase by the helicase loader DnaC, followed by recruitment of primase DnaG and the DNA polymerase III holoenzyme to establish the replisome. Once initiation has occurred, new rounds of replication begin only after cellular mechanisms reset the active DnaA pool.

Regulation of oriC activity ensures the chromosome replicates once per cell cycle. Factors such as RIDA (regulating the inactivation of DnaA), the datA locus, and nucleoid-associated proteins influence the timing and frequency of initiation, coordinating replication with cell growth and division.