It was demonstrated previously that a deoxyribophosphodiesterase (dRpase) activity is associated with the DNA repair enzyme exonuclease I, and that this activity is stimulated by the addition of the E. coli single-stranded DNA-binding protein (Ssb). This activity catalyzes the release of deoxyribose-phosphate groups at apurinic/apyrimidinic (AP) sites in the DNA that have been cleaved by the action of an AP endonuclease. We have now used the yeast two-hybrid system to demonstrate that a protein-protein interaction occurs between exonuclease I and Ssb. When the E. coli ssb gene was fused in frame to the DNA-activating domain of the GAL4 transcriptional activator and the exonuclease I gene was fused in frame to the DNA-binding domain, a functional GAL4 transcriptional activator was produced as determined by growth of yeast on selective medium and the measurement of β-galactosidase activity. We have also demonstrated that Ssb can stimulate the dRpase activity of exonuclease I using double-stranded bacteriophage M13 DNA containing several strand interruptions at incised AP sites. These results suggest that Ssb may be required for efficient base-excision repair in bacteria.

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