The specificity of lambda exonuclease. Interactions with single-stranded DNA.

The lambda exonuclease, an enzyme that has been implicated in genetic recombination, rapidly and processively degrades native DNA, starting at the 5' terminus. The enzyme will also degrade the 5'-terminated strand at a single-stranded branch. The experiments reported here reveal various interactions of the enzyme with single-stranded DNA. The rate of digestion is related inversely to the length of single strands. Chains of 100 nucleotides are digested at about 10% the rate of digestion of native DNA. Digestion of the single-stranded ends of lambda DNA does not appear to occur processively. The enzyme binds to circular as well as linear single strands and the affinity for single strands is also related inversely to the chain length. In an equimolar mixture of single- and double-stranded DNA the action of lambda exonuclease on the latteris about half-inhibited. At 20 degrees the initiation of digestion at the 5' terminus of duplex DNA is blocked sterically when such DNA has 3'-terminal single strands that are longer than 100 nucleotides. Information about these properties is important for the practical use of lambda exonuclease as well as for reflections on the role of the enzyme in genetic recombination.