In vivo Requirement for RecJ, ExoVII, ExoI, and ExoX in Methyl-Directed Mismatch Repair

• Published in: Proceedings of the National Academy of Sciences - PNAS 2001-06, Vol.98 (12), p.6765-6770
• Main Authors: Burdett, Vickers, Baitinger, Celia, Viswanathan, Mohan, Lovett, Susan T., Modrich, Paul
• Format: Article
• Language: English
• Subjects: 2-aminopurine; Bacterial Proteins - physiology; Base Pair Mismatch; Biochemistry; Biological Sciences; Biosynthesis; Cell growth; Chromosomes; Cold Temperature; Complementation; DNA; DNA damage; DNA mismatch repair; DNA Repair; DNA Repair Enzymes; DNA-Binding Proteins - physiology; Endodeoxyribonucleases - physiology; Escherichia coli; Escherichia coli Proteins; Exodeoxyribonucleases - physiology; exonuclease I; exonuclease VII; exonuclease X; Genetic mutation; mismatch repair; Mutation; mutH gene; mutL gene; mutS gene; Phenotypes; RecJ protein; SOS response
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.121183298

Biochemical studies with model DNA heteroduplexes have implicated RecJ exonuclease, exonuclease VII, exonuclease I, and exonuclease X in Escherichia coli methyl-directed mismatch correction. However, strains deficient in the four exonucleases display only a modest increase in mutation rate, raising questions concerning involvement of these activities in mismatch repair in vivo. The quadruple mutant deficient in the four exonucleases, as well as the triple mutant deficient in RecJ exonuclease, exonuclease VII, and exonuclease I, grow poorly in the presence of the base analogue 2-aminopurine, and exposure to the base analogue results in filament formation, indicative of induction of SOS DNA damage response. The growth defect and filamentation phenotypes associated with 2-aminopurine exposure are effectively suppressed by null mutations in mutH, mutL, mutS, or uvrD/mutU, which encode activities that act upstream of the four exonucleases in the mechanism for the methyl-directed reaction that has been proposed based on in vitro studies. The quadruple exonuclease mutant is also cold-sensitive, having a severe growth defect at 30°C. This phenotype is suppressed by a uvrD/mutU defect, and partially suppressed by mutH, mutL, or mutS mutations. These observations confirm involvement of the four exonucleases in methyl-directed mismatch repair in vivo and suggest that the low mutability of exonuclease-deficient strains is a consequence of under recovery of mutants due to a reduction in viability and/or chromosome loss associated with activation of the mismatch repair system in the absence of RecJ exonuclease, exonuclease VII, exonuclease I, and exonuclease X.