A plant 3 '-phosphoesterase involved in the repair of DNA strand breaks generated by oxidative damage

Two novel, structurally and functionally distinct phosphatases have been id entified through the functional complementation, by maize cDNAs, of an Esch erichia coli diphosphonucleoside phosphatase mutant strain. The first, ZmDP 1, is a classical Mg2+-dependent and Li+-sensitive diphosphonucleoside phos phatase that dephosphorylates both 3'-phosphoadenosine 5'-phosphate (3'-PAP ) and 2'-PAP without any discrimination between the 3'- and 2'-positions. T he other, ZmDP2, is a distinct phosphatase that also catalyzes diphosphonuc leoside dephosphorylation, but with a 12-fold lower Li+ sensitivity, a stro ng preference for 3'-PAP, and the unique ability to utilize double-stranded DNA molecules with 3'-phosphate- or 3'-phosphoglycolate-blocking groups as substrates, Importantly, ZmDP2, but not ZmDP1, conferred resistance to a D NA repair-deficient E, coli strain against oxidative DNA-damaging agents ge nerating 3'-phosphate- or 3'-phosphoglycolate-blocked single strand breaks. ZmDP2 shares a partial amino acid sequence similarity with a recently iden tified human polynucleotide kinase 3'-phosphatase that is thought to be inv olved in DNA repair, but is devoid of 5'-kinase activity. ZmDP2 is the firs t DNA 3'-phosphoesterase thus far identified in plants capable of convertin g 3'-blocked termini into priming sites for reparative DNA polymerization.