8.12:
Mismatch Repair
Mismatched base pairs, such as adenine with cytosine, can sometimes escape proofreading during replication and are later fixed through the process of mismatch repair.
In E. coli, the mismatch recognition protein, MutS, identifies mismatched bases due to their abnormal structure.
It then recruits MutL and forms the MutS-MutL complex. This activated complex stimulates MutH, an endonuclease, to nick the newly synthesized strand.
Genomic DNA in E. coli usually has methyl groups attached at specific sites. However, there is a certain time delay before a newly synthesized strand is methylated.
This allows the repair complex to identify the new strand based on its methylation status, and MutH nicks it at the nearest unmethylated site, while the template strand remains unchanged.
A helicase unwinds the cleaved DNA segment from the template strand. An exonuclease hydrolyses the erroneous region, thus, leaving a gap in the new strand.
Finally, DNA polymerase fills the gap with the correct nucleotides, and DNA ligase seals the nick, repairing the error.
8.12:
Mismatch Repair
Organisms are capable of detecting and fixing nucleotide mismatches that occur during DNA replication. This sophisticated process requires identifying the new strand and replacing the erroneous bases with correct nucleotides. Mismatch repair is coordinated by many proteins in both prokaryotes and eukaryotes.
The Mutator Protein Family Plays a Key Role in DNA Mismatch Repair
The human genome has more than 3 billion base pairs of DNA per cell. Prior to cell division, that vast amount of genetic information needs to be replicated. Despite the proofreading ability of the DNA polymerase, a copying error occurs approximately every 1 million base pairs. One type of error is the mismatch of nucleotides, for example, the pairing of A with G or T with C. Such mismatches are detected and repaired by the Mutator protein family. These proteins were first described in the bacteria Escherichia coli (E. coli), but homologs appear throughout prokaryotes and eukaryotes.
Mutator S (MutS) initiates the mismatch repair (MMR) by identifying and binding to the mismatch. Subsequently, MutL identifies which strand is the new copy. Only the new strand requires fixing, while the template strand needs to remain intact. How can the molecular machinery identify the newly synthesized strand of DNA?
Newly Synthesized Strands of DNA Differ from Their Template Strand
In many organisms, cytosine and adenine bases of the new strand receive a methyl group sometime after replication. Therefore, Mut proteins identify new strands by recognizing sequences that have not yet been methylated. Additionally, the newly synthesized strand in eukaryotes is more likely to have small breaks, also called DNA nicks. The MMR proteins can thus identify the nicked strand and target it for repair.
After identification of the new strand, nuclease enzymes cut the affected region and excise the incorrect nucleotides. Next, DNA polymerase fills in the correct nucleotides, and DNA ligase seals the sugar-phosphate backbone of the DNA, thereby completing the mismatch repair process.
Defects in the Mismatch Repair Mechanism Can Cause Cancer
The human homolog of MutS is called Mutator S homolog 2 (MSH2). If MSH2 function is compromised, point mutations and frameshift mutations throughout the genome are not properly repaired. Consequently, humans carrying a single copy of such a compromised MSH2 have a higher likelihood of developing cancer.
Unrepaired Mutations Fuel Adaptation
Would it be best if MMR never missed a mismatch? Even low mutation rates can cause a problem for an organism. However, mutations also contribute to genetic variation in a population. For instance, a permissive mismatch repair system in a bacterium can lead to the mutation of a gene that confers antibiotic resistance, thereby increasing the chances of bacterial survival and reproduction when exposed to antibiotics. This is great news for the bacterial population but bad news for humans that rely on antibiotics to combat infectious diseases.