An exonuclease that acts preferentially on linear dsDNA, processively degrading it from 5'-3', releasing 5'-phosphomononucleotides. Initiates on 5'-phosphate and 5'-hydroxyl ends. Also acts on linear ssDNA, nicked DNA and RNA. ATP enhances but is not necessary for exonuclease activity; has ATPase activity that is not stimulated by DNA (PubMed:7836278). The old protein kills E.coli recB and recC mutants and interferes with phage lambda growth (PubMed:4913204). Both the exonuclease and ATPase activities are required in vivo (PubMed:32009148). Probably interferes with lambda phage by degrading its linear DNA (Probable). Isolated as a mutant able to lysogenize E.coli strain C cells normally not susceptible to lysis by phage P2 (PubMed:4884707). Exonucleolytic cleavage in the 5'- to 3'-direction to yield nucleoside 5'-phosphates. Mg(2+) gave the most activity in a maltose-binding protein fusion. Optimum pH is 7.2-8.8. Belongs to the class 1 OLD nuclease family.