RecA-independent single-stranded DNA oligonucleotide-mediated mutagenesis

The expression of Beta, the single-stranded annealing protein (SSAP) of bacteriophage λ in Escherichia coli promotes high levels of oligonucleotide (oligo)-mediated mutagenesis and offers a quick way to create single or multiple base pair insertions, deletions, or substitutions in the bacterial chromosome. High rates of mutagenesis can be obtained by the use of mismatch repair (MMR)-resistant mismatches or MMR-deficient hosts, which allow for the isolation of unselected mutations. It has recently become clear that many bacteria can be mutagenized with oligos in the absence of any SSAP expression, albeit at a much lower frequency. Studies have shown that inactivation or inhibition of single-stranded DNA (ssDNA) exonucleases in vivo increases the rate of SSAP-independent oligo-mediated mutagenesis. These results suggest that λ Beta, in addition to its role in annealing the oligo to ssDNA regions of the replication fork, promotes high rates of oligo-mediated mutagenesis by protecting the oligo from destruction by host ssDNA exonucleases.