Isolation of Exonuclease VIII: The Enzyme Associated with the sbcA Indirect Suppressor

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Abstract
RecB and/or recC deficiency in Escherichia coli K-12 is indirectly suppressed by the presence of sbcA(-) mutations. sbcA(-) strains contain an increased level of an ATP-independent nuclease. Genetic and enzymatic tests indicate that this activity is not exonuclease III, exonuclease V (recB-recC nuclease), DNA polymerase I, or lambda exonuclease. This new enzyme (exonuclease VIII) has been purified 750-fold and shows a striking preference for double-stranded DNA over heat-denatured DNA. It does not act endonucleolytically on closed circular, single-stranded DNA as exonuclease V does. It also lacks a 3'-phosphatase function. Analysis on sodium dodecyl sulfate-polyacrylamide gels indicates that exonuclease VIII is not present in unsuppressed (sbcA(+)) strains. It is thought that sbcA determines some type of control function; the structural gene for exonuclease VIII is denoted by recE.