Mismatches introduced during DNA replication are detected and repaired efficiently by the “Mut” system of E. coli.  (A) Please outline the steps in mismatch detection and repair by this system.  (B) What is the historical reason for naming these genes “Mut” in the first place? (C) How might you identify bacterial strains with defects in the “Mut” system? (D) It has been observed that recombination-deficient mutations are usually lethal when they are combined with mutations in the mismatch repair pathway you just described.  Why is that?

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Asked Apr 7, 2019
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Mismatches introduced during DNA replication are detected and repaired efficiently by the “Mut” system of E. coli.  (A) Please outline the steps in mismatch detection and repair by this system.  (B) What is the historical reason for naming these genes “Mut” in the first place? (C) How might you identify bacterial strains with defects in the “Mut” system? (D) It has been observed that recombination-deficient mutations are usually lethal when they are combined with mutations in the mismatch repair pathway you just described.  Why is that?

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Expert Answer

Step 1

(A) A repair mechanism of DNA (deoxyribonucleic acid) called as MMR (DNA mismatch repair) involves removal of incorrectly inserted base pairs and mis-incorporated bases that can occur during replication and/or recombination. The three essential proteins in coli that are involved in the said DNA repair mechanism are MutS (Mutator S), MutH (Mutator H), and MutL (Mutator L). The major steps involved in this mechanism are outlined as below:

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Step 2

(B) The evidence for DNA mismatch repair (MMR) system was tested and identified in Streptococccus pneumoniae for the first time. This system is highly conserved from the prokaryotes to the eukaryotes. When experiments were performed on Escherichia coli, the production of hypermutable strains in mutationally inactivated genes was seen. These genes therefore came to be known as “Mut” proteins because they were a major part of the repair mechanism.

Step 3

(C) The bacterial strains with a defective methyl-directed mismatch repair system or “Mut” system will depict hypermutation and will probably be pathogenic in nature. The defect in this system has been attributed largely to the frequent use of drugs that imparted resistance in the strains by causing mutations. In laboratory, the defective “Mut” system or mutation in the bacterial strain can be identified using standard tests like disc diffusion or aga...

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