We have recently described the presence of a high proportion of Pseudomonas 
aeruginosa isolates (20%) with an increased mutation frequency (mutators) in the 
lungs of cystic fibrosis (CF) patients. In four out of 11 independent P. 
aeruginosa strains, the high mutation frequency was found to be complemented 
with the wild-type mutS gene from P. aeruginosa PAO1. Here, we report the 
cloning and sequencing of two additional P. aeruginosa mismatch repair genes and 
the characterization, by complementation of deficient strains, of these two 
putative P. aeruginosa mismatch repair genes (mutL and uvrD). We also describe 
the alterations in the mutS, mutL and uvrD genes responsible for the mutator 
phenotype of hypermutable P. aeruginosa strains isolated from CF patients. Seven 
out of the 11 mutator strains were found to be defective in the MMR system (four 
mutS, two mutL and one uvrD). In four cases (three mutS and one mutL), the genes 
contained frameshift mutations. The fourth mutS strain showed a 3.3 kb insertion 
after the 10th nucleotide of the mutS gene, and a 54 nucleotide deletion between 
two eight nucleotide direct repeats. This deletion, involving domain II of MutS, 
was found to be the main one responsible for mutS inactivation. The second mutL 
strain presented a K310M mutation, equivalent to K307 in Escherichia coli MutL, 
a residue known to be essential for its ATPase activity. Finally, the uvrD 
strain had three amino acid substitutions within the conserved ATP binding site 
of the deduced UvrD polypeptide, showing defective mismatch repair activity. 
Interestingly, cells carrying this mutant allele exhibited a fully active 
UvrABC-mediated excision repair. The results shown here indicate that the 
putative P. aeruginosa mutS, mutL and uvrD genes are mutator genes and that 
their alteration results in a mutator phenotype.