nicks the unmethylated strand, so only newly synthesized DNA in the vicinity of the mismatch is removed and replaced (Figure 9-Sb). Methylation is therefore a "memory" device that enables the E. coii repair system to retrieve the correct sequence from the parental strand if an error has been made during replication.
Different exonucleases are used to remove single-stranded DNA between the nick created by MutH and the mismatch, depending on whether MutH cuts the DNA on the 5' or the 3' side of the misincorpo-raled nucleotide. If the DNA is cleaved on the 5' side of the mismatch, then exonuclease VII or RecJ, which degrade DNA in a direction, remove the stretch of DNA from the MutH-induced cut through the mis-incorporated nucleotide. Conversely, if the nick is on the 3' side of the mismatch, then the DNA is removed by exonuclease I, which degrades DNA in a 3'—'>5' direction. As we have seen, alter removal of the mismatched base, DNA Pol HI fills in the missing sequence (Figure 9-6).
Eukaryotic cells also repair mismatches and do so using homologs to MutS (called MSH proteins for MutS homologs) and MutL (called MLH and PMS). indeed, enkaryotes have multiple MutS-like proteins with different specificities. For example, one is specific for simple mismatches, whereas another recognizes small insertions or deletions resulting from "slippage" during DNA replication. Dramatic evidence diat mismatch repair plays a critical role in higher organisms came from the discovery that a genetic predisposition to colon cancer (hereditary nonpolyposis colorectal cancer) is due to a mutation in the genes for human homologs of MutS (specifically the MSH2 homolog) and MutL,
FIGURE 9-5 Dam methylation at replication fork (a) Replication generates hemimethylated DNA in £ coir (b) MutH makes incision in unmethylated daughter strand.
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