The initiation of DNA replication is tightly controlled in all organisms. Discuss how this is accomplished in E. coli. Consider the following: a) What protein is responsible for initiating DNA replication

DnaA is responsible for initiating DNA replication

Explanation:

DNA polymerases are responsible for synthesizing new strands of DNA and can only perform its activity when the double helix of the DNA has been unwound. The area where replication begins is called the replication fork. The DNA replication involves three stages namely; initiation, elongation and termination.

The initiation in E. coli is at a specific nucleotide called the origin of chromosomal replication (oriC) which has 9bp binding sites for the initiator protein Dna A. Over 40 DnaA proteins bind oriC causing negative supercoiling of the DNA to melt three 13bp AT-rich repeat sequences for another protein called DnaB to bind. This new protein is a DNA helicase responsible for the unwinding of the DNA double helix strand. After unwinding, single-stranded binding proteins (SSB) helps keep the DNA strands apart preventing it form renaturing.  

DNA primase enzyme synthesizes RNA primers which begins to bind nucleotides on the leading strands in a 3’ – 5’ direction.  

Replication in E. coli are of two patterns;  

•Bi-directional replication (In E. coli with circular chromosome, the replication would be bidirectional from the origin of replication. Two replication forks on each chromosome would move in opposite directions held together by two Tau protein subunits. This would lead to formation of theta structures. During replication, synthesis occurs on each template strand in both leading and lagging fashion and it is rapid. The enzyme Polymerase iii (POL III) adds nucleotide to the growing strand at a very fast rate of about 1000 nucleotide per second. This sums the entire E. coli replication to 40minutes. When the replication time is shorter, it has been found that such E. coli strains possess multiple replication forks (start of a new replication before the end of an initial replication cycle).

•Rolling circle replication occurs during conjugation (genetic exchange)

DnaA is the main protein at the beginning of E. coli DNA replication. It all starts at the origin of the oriC replication.

Explanation:

The DnaA protein recognizes 9 bp sequences and produces a nucleoprotectic structure, coiled by DNA; the DNA will facilitate the opening and there the DnaC and DnaB join. DnaB is a helicase that forms the complex with DnaC and this complex interacts with the DnaA protein.

DnaB opens the DNA. There are two types of helicases that advance through DNA. In E. coli, 12 helicases have been described. DnaB is type 5’→ 3’. In the oriC the initiation complex formed by DnaA, DnaB and DnaC is coupled, this complex undergoes a remodeling in its structure, DnaC puts DnaB on the fork but to serve as a helicase it has to leave. At that time the replication machinery is assembled.

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