Pseudomonas aeruginosa colonizes the respiratory tract of Cystic Fibrosis patients, where a high proportion of hypermutable variants along with mucoid, alginate-overproducing variants, emerge leading to chronic infection and poor prognosis. Conversion to mucoidy is usually caused by a G deletion within a homopolymeric run of G in the mucA gene (mucA22). Studying the emergence of mucoids in wild type and hypermutable mutS deficient strains of P. aeruginosa PAO1, we observed a 37 fold increase in the frequency of mucoid variants emerged from the mutS mutant. While only 12% of mucoids from the wild type strain carried a mucA22 allele, this proportion reached 64% for the mutS strain. Since in Escherichia coli Pol IV (dinB) is involved in the generation of -1 deletions in homopolymeric runs, we analyzed emergence of mucoid variants in strains of P. aeruginosa dinB and mutS dinB double mutants. Emergence of mucoids suffered a 3 fold decrease in the dinB strain, and only a 2 fold increase was observed in the mutS dinB double mutant respect to the wild type strain. Furthermore, over-expression of Pol IV in the mutS strain resulted in a 110 fold increase in the emergence of mucoid variants. Thus we provide the first evidence of the mutagenic activity of Pol IV in P. aeruginosa and show that in mucoid conversion, hypermutability of mucA is mainly due to Pol IV activity.
