Short single-stranded gaps in circular DNA molecules can be generated enzymatically, often at predetermined sites. These can serve as targets for in vitro mutagenesis procedures that result in alterations in nucleotide sequence within or very near the gap. Deamination of unpaired cytosine residues with sodium bisulfite has been used to induce mutations in the BglI restriction site of SV40 DNA and within defined regions of the beta-lactamase gene on pBR322. A new method of induction of mutations at gaps, called "gap misrepair," has been developed; it was used to cause changes at the HindIII and C1aI restriction sites on pBR322 DNA. Gap misrepair reactions using DNA polymerase I of Micrococcus luteus in the presence of T4 DNA ligase and three of the four deoxynucleoside triphosphates yielded all three possible substitutions for adenine and cytosine residues in the DNA.
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