Identification and expression of the TREX1 and TREX2 cDNA sequences encoding mammalian 3'-->5' exonucleases.

The 3'-->5' exonucleases catalyze the excision of nucleoside monophosphates from the 3' termini of DNA. We have identified the cDNA sequences encoding two 3'-->5' exonucleases (TREX1 and TREX2) from mammalian cells. The TREX1 and TREX2 proteins are 304 and 236 amino acids in length, respectively. Analysis of the TREX1 and TREX2 sequences identifies three conserved motifs that likely generate the exonuclease active site in these enzymes. The specific amino acids in these three conserved motifs suggest that these mammalian exonucleases are most closely related to the proofreading exonucleases of the bacterial replicative DNA polymerases and the RNase T enzymes. Expression of TREX1 and TREX2 in Escherichia coli demonstrates that these recombinant proteins are active 3'-->5' exonucleases. The recombinant TREX1 protein was purified, and exonuclease activity was measured using single-stranded, partial duplex, and mispaired oligonucleotide DNA substrates. The greatest activity of the TREX1 protein was detected using a partial duplex DNA containing five mispaired nucleotides at the 3' terminus. No activity was detected using single-stranded RNA or an RNA-DNA partial duplex. Identification of the TREX1 and TREX2 cDNA sequences provides the genetic tools to investigate the physiological roles of these exonucleases in mammalian DNA replication, repair, and recombination pathways.