Mismatch DNA repair

[hullah]

Base excision and nucleotide excision based DNA repair in E. coli use DNA polymerase I but the DNA mismatch repair of the daughter strand (unmethylated) uses DNA polymerase III to fill up the excised DNA strand. I can not explain the basis of using two different polymerases to perform apparently the same function (fill up a single strand gap resulting from excision). I know that Pol I has both 5' to 3' and 3' to 5' exonuclease activity whereas Pol III has only 3' to 5' exonuclease activity.

[JackBean]

an interesting question, hopefully someone could answer it

[aptitude]

DNA polymerase III proofreads as it elongates the strand, using a primer with a 3' hydroxyl. DNA polymerase I proofreads as it replaces the primer.

[JackBean]

that's right, but what's the reasoning to use two different Pols for base repair?

[TheWalkingGlitch]

Increased efficiency? The processes of strand elongation and primer replacement are spatially/temporally separated, it's faster if 2 polymerases do it at once.

Another way of looking at it... proofreading is such an important ability that if one polymerase is defective, the other one still has it.

From an evolutionary standpoint, the redundant function arose as a result of random mutation, and it increased fitness so it became dominant in the population.

Besides, does there have to be a reason for everything? As the saying goes, "If it ain't broken, don't fix it"