Bidirectionally degrades single-stranded DNA into large acid-insoluble oligonucleotides, which are then degraded further into small acid-soluble oligonucleotides. It can degrade 3' or 5' ss regions extending from the termini of duplex DNA molecules and displaced ss regions. It can also excise thymine dimers in vitro (PubMed:4602029, PubMed:4602030, PubMed:22718974) (Probable). ssDNA-binding requires both subunits (PubMed:22718974). Required for production of the mature 5'-end of retron Ec78 or Ec83 msDNA. Overproduction of this subunit in the absence of an equivalent quantity of the small subunit is toxic, causing cell elongation and chromosome fragmentation or loss; its toxicity is mostly suppressed by RecA (PubMed:26626352). Exonucleolytic cleavage in either 5'- to 3'- or 3'- to 5'-direction to yield nucleoside 5'-phosphates. Does not require a metal cofactor. Optimum pH is 7.8 to 7.9. Heterooligomer composed of two different subunits with an approximate ratio of 4:1 for small to large subunit (Probable). Also estimated to have a 6:1 ration for small to large subunits (Probable). Predicted to have 4 domains. The N-terminus (about residues 1-103) binds ssDNA and is required to bind the small subunit; it probably has an OB-fold. The predicted catalytic domain is residues 104-266. Three alpha-helices are predicted in the C-terminal region (residues 267-301, 307-349 and 353-393), their removal singly or in pairs reduces small subunit-binding; none of the deletions have exonuclease activity. The extreme C-terminus (394-456) is required for exonuclease activity (PubMed:22718974). The N-terminus (residues 1-257) at low levels does not confer processing of msDNA and at higher levels is lethal. Lethality of this fragment is not counteracted by the small subunit (PubMed:26626352). No longer processes msDNA correctly (when retron Ec78 or Ec83 are expressed in the strain). Belongs to the XseA family.