Mutation rate, spectrum, topology, and context-dependency in the DNA mismatch repair-deficient Pseudomonas fluorescens ATCC948

High levels of genetic diversity exist among natural isolates of the bacterium Pseudomonas fluorescens, and are especially elevated around the replication terminus of the genome, where strain-specific genes are found. In an effort to understand the role of genetic variation in the evolution of Pseudomonas, we analyzed 31,106 base substitutions from 45 mutation accumulation lines of P. fluorescens ATCC948, naturally deficient for mismatch repair, yielding a base-substitution mutation rate of 2.34×10^-8 per site per generation (SE: 0.01×10^-8) and a small-insertion-deletion mutation rate of 1.65×10^-9 per site per generation (SE: 0.03×10^-9). We find that the spectrum of mutations in prophage regions, which often contain virulence factors and antibiotic resistance, is highly similar to that in the intergenic regions of the host genome. Our results show that themutation rate varies around the chromosome, with the lowest mutation rate foundnear the originof replication. Consistent with observations fromother studies, wefind that site-specificmutation rates are heavily influenced by the immediately flanking nucleotides, indicating thatmutations are context dependent.