| Summary: | 碩士 === 國立臺灣大學 === 醫事技術學研究所 === 90 === The Escherichia coli methyl-directed, MutHLS-dependent mismatch repair system maintains genetic integrity by correcting DNA biosynthetic errors and ensuring the fidelity of homologous genetic recombination.The repair is directed on the newly replicated strand that is transiently unmethylated at d(GATC) sequences, which are subsequently methylated by the dam gene product.Initiation of repair involves the mismatch-provoked incision of the unmodified strand at a hemimethylated d(GATC) sequences. The resulting strand break,which can occur either 3’ or 5’ to the mismatch on the unmethylated strand,suffices to target correct on to this strand. The Escherichia coli mismatch repair system can repair all types of mismatch except C-C mismatch.Since the results described to date suggested that there was no C-C mismatch specific repair system in E. coli, it’s interesting for us to test whether a mismatch near a C-C
mismatch could provoke the repair of both heterologies (corepair phenomenon).
We constructed a series of f1-P hemimethylated heteroduplex substrates containing both single base mismatch and C-C mismatch. The C-C mismatch locates 64 bases upstream or downstream to the single base mismatch. Repair efficiency of these substrates were determined by using an in vitro assay. Our results showed that C-C mismatch in the vicinity of the mispair base was corepaired by E.coli cell extracts, especially for G/G, T/G, A/A those strongly repaired mismatches, being strand specific and highly biased to the unmethylated strand. The in vitro activity was dependent on products of mutS, mutH,
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