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). Required for production of the mature 5'-end of retron Ec78 or Ec83 msDNA. When in excess of the large subunit, counteracts the large subunit's toxicity (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). No longer processes msDNA correctly (when retron Ec78 or Ec83 are expressed in the strain). Belongs to the XseB family.