Oligonucleotides stimulate genomic alterations of Legionella pneumophila

Genetic variation generates diversity in all kingdoms of life. The corresponding mechanisms can also be harnessed for laboratory studies of fundamental cellular processes. Here we report that oligonucleotides (oligos) generate mutations on the Legionella pneumophila chromosome by a mechanism that requires homologous DNA, but not RecA, RadA or any known phage recombinase. Instead we propose that DNA replication contributes, as oligo-induced mutagenesis required ≥ 21 nucleotides of homology, was strand-dependent, and was most efficient in exponential phase. Mutagenesis did not require canonical 5' phosphate or 3' hydroxyl groups, but the primosomal protein PriA and DNA Pol I contributed. After electroporation, oligos stimulated excision of 2.1 kb of chromosomal DNA or insertion of 18 bp, and non-homologous flanking sequences were also processed. We exploited this endogenous activity to generate chromosomal deletions and to insert an epitope into a chromosomal coding sequence. Compared with Escherichia coli, L. pneumophila encodes fewer canonical single-stranded exonucleases, and the frequency of mutagenesis increased substantially when either its RecJ and ExoVII nucleases were inactivated or the oligos modified by nuclease-resistant bases. In addition to genetic engineering, oligo-induced mutagenesis may have evolutionary implications as a mechanism to incorporate divergent DNA sequences with only short regions of homology.