Describe the process of DNA replication in Escherichia coli

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Initiation of Replication and the Bacterial Origin

DNA replication in E. coli is bi-directional and originates at a single origin of replication (OriC). The initiation of replication is mediated by DnaA, a protein that binds to a region of the origin known as the DnaA box. In E. coli, there are 5 DnaA boxes, each of which contains a highly conserved 9bp consensus sequence 5' - TTATCCACA - 3'. Binding of DnaA to this region causes it to become negatively supercoiled. Following this, a region of OriC upstream of the DnaA boxes (known as DnaB boxes) become melted. There are three of these regions, and each are 13bp long, and AT rich (which obviously facilitates melting because less energy is required to break the two hydrogen bonds that form between A and T nucleotides). This region has the consensus sequence 5' - GATCTNTTNTTTT - 3. Melting of the DnaB boxes requires ATP (which is hydrolyzed by DnaA). Following melting, DnaA recruits a hexameric helicase (six DnaB proteins) to opposite ends of the melted DNA. This is where the replication fork will form. Recruitment of helicase requires six DnaC proteins, each of which is attached to one subunit of helicase. Once this complex is formed, an additional five DnaA proteins bind to the original five DnaA proteins to form five DnaA dimers. DnaC is then released, and the prepriming complex is complete. In order for DNA replication to continue, SSB is needed to prevent the single strands of DNA from forming any secondary structures and to prevent them from reannealing, and DNA gyrase is needed to relieves the stress (by creating negative supercoils) created by the action of DnaB helicase. The unwinding of DNA by DnaB helicase allows for primase (DnaG) and RNA polymerase to prime each DNA template so that DNA synthesis can begin.

Termination of Replication


Termination of DNA replication in E. coli is completed through the use of termination sequences and the Tus protein. These sequences allow the two replication forks to pass through in only one direction, but not the other. In order to slow down and stop the movement of the replication fork in the termination region of the E. coli chromosome, the Tus protein is required. This protein binds to the termination sites, and prevents DnaB from displacing DNA strands. However, these sequences are not required for termination of replication.


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http://www.maxanim.com/genetics/Dna%20Replication/Dna%20Replication.htm

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2007-03-21 10:07:14 補充：
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