Structure and Expression of the TREX1 and TREX2 3′→5′ Exonuclease Genes

The TREX1 and TREX2 genes encode mammalian 3′→5′ exonucleases. Expression of the TREX genes in human cells was investigated using a reverse transcription-polymerase chain reaction strategy. Our results show that TREX1 and TREX2 are expressed in all tissues tested, providing direct evidence for the expression of these genes in human cells. Potential transcription start sites are identified for the TREX genes using rapid amplification of cDNA ends to recover the 5′-flanking regions of the TREX transcripts. The 5′-flanking sequences indicate transcription initiation from consensus putative promoters identified −140 and −650 base pairs upstream of the TREX1 open reading frame (ORF) and −623 and −753 base pairs upstream of the TREX2 ORF. Novel TREX1 and TREX2 cDNAs are identified that contain protein-coding sequences generated from exons positioned in genomic DNA up to 18 kilobases 5′ to the TREX1 ORF and up to 25 kilobases 5′ to the TREX2 ORF. These novel cDNAs and sequences in the GenBank™ data base indicate that transcripts containing the TREX1 and TREX2 ORFs are produced using a variety of mechanisms that include alternate promoter usage, alternative splicing, and varied sites for 3′ cleavage and polyadenylation. These initial studies have revealed previously unrecognized complexities in the structure and expression of the TREX1 and TREX2 genes. The TREX1 and TREX2 genes encode mammalian 3′→5′ exonucleases. Expression of the TREX genes in human cells was investigated using a reverse transcription-polymerase chain reaction strategy. Our results show that TREX1 and TREX2 are expressed in all tissues tested, providing direct evidence for the expression of these genes in human cells. Potential transcription start sites are identified for the TREX genes using rapid amplification of cDNA ends to recover the 5′-flanking regions of the TREX transcripts. The 5′-flanking sequences indicate transcription initiation from consensus putative promoters identified −140 and −650 base pairs upstream of the TREX1 open reading frame (ORF) and −623 and −753 base pairs upstream of the TREX2 ORF. Novel TREX1 and TREX2 cDNAs are identified that contain protein-coding sequences generated from exons positioned in genomic DNA up to 18 kilobases 5′ to the TREX1 ORF and up to 25 kilobases 5′ to the TREX2 ORF. These novel cDNAs and sequences in the GenBank™ data base indicate that transcripts containing the TREX1 and TREX2 ORFs are produced using a variety of mechanisms that include alternate promoter usage, alternative splicing, and varied sites for 3′ cleavage and polyadenylation. These initial studies have revealed previously unrecognized complexities in the structure and expression of the TREX1 and TREX2 genes. exonuclease GenBank™ accession no. reverse transcription polymerase chain reaction open reading frame expressed sequence tags base pair(s) kilobase(s) group of overlapping clones acute myeloblastic leukemia Neural Network Promoter Prediction genome survey sequence The multistep processes of DNA replication, repair, and genetic recombination often require the excision of 3′ nucleotides to generate DNA 3′ termini suitable for subsequent metabolic steps. The apparent diversity of proteins containing 3′ → 5′ exonuclease activity likely reflects the different requirements for these enzymes in the maintenance of the human genome. In some cases these exonucleases are found in large proteins that contain multiple catalytic and functional properties. The 3′ → 5′ proofreading exonucleases are functional domains in the mammalian DNA polymerases δ (1Chung D.W. Zhang J. Tan C.-K. Davie E.W. So A.G. Downey K.M. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 11197-11201Crossref PubMed Scopus (58) Google Scholar), ε (2Kesti T. Frantti H. Syväoja J.E. J. Biol. Chem. 1993; 268: 10238-10245Abstract Full Text PDF PubMed Google Scholar), and γ (3Ropp P.A. Copeland W.C. Genomics. 1996; 36: 449-458Crossref PubMed Scopus (243) Google Scholar). These proofreading enzymes remove incorrectly polymerized nucleotides during DNA synthesis and minimize the incorporation of mismatches into the genome. The Werner syndrome protein (WRN) contains a 3′ → 5′ exonuclease activity in one functional domain and a 3′ → 5′ DNA helicase activity in another (4Shen J.C. Gray M.D. Oshima J. Ashwini S.K. Fry M. Loeb L.A. J. Biol. Chem. 1998; 273: 34139-34144Abstract Full Text Full Text PDF PubMed Scopus (221) Google Scholar, 5Yu C.E. Oshima J. Fu Y.H. Wijsman E.M. Hisama F. Alisch R. Matthews S. Nakura J. Miki T. Ouais S. Martin G.M. Mulligan J. Schellenberg G.D. Science. 1996; 272: 258-262Crossref PubMed Scopus (1489) Google Scholar). Deficiencies in the WRN protein increase genomic instability (6Shen J.C. Loeb L.A. Trends in Genetics. 2000; 16: 213-220Abstract Full Text Full Text PDF PubMed Scopus (148) Google Scholar). The multifunctional p53 protein contains a 3′ → 5′ exonuclease localized to the central core domain (7Mummenbrauer T. Janus F. Müller B. Wiesmüller L. Deppert W. Grosse F. Cell. 1996; 85: 1089-1099Abstract Full Text Full Text PDF PubMed Scopus (246) Google Scholar). This core region in p53 also contains the sequence-specific DNA binding domain that functions in cell-cycle checkpoint control in mammalian cells (8Albrechtsen N. Dornreiter I. Grosse F. Kim E. Wiesmuller L. Deppert W. Oncogene. 1999; 18: 7706-7717Crossref PubMed Scopus (151) Google Scholar). The hRAD1 (Ustilago maydis REC1) and hRAD9 are human homologues of yeast DNA damage checkpoint response proteins (9Udell C.M. Lee S.K. Davey S. Nucleic Acids Res. 1998; 26: 3971-6397Crossref PubMed Scopus (35) Google Scholar). These proteins also contain 3′ → 5′ exonuclease activities (10Thelen M.P. Onel K. Holloman W.K. J. Biol. Chem. 1994; 269: 747-754Abstract Full Text PDF PubMed Google Scholar, 11Parker A.E. Van de Weyer I. Laus M.C. Oostveen I. Yon J. Verhasselt P. Luyten W.H. J. Biol. Chem. 1998; 273: 18332-18339Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar, 12Bessho T. Sancar A. J. Biol. Chem. 2000; 275: 7451-7454Abstract Full Text Full Text PDF PubMed Scopus (69) Google Scholar). The yeast mre11 mutant is defective in recombinational DNA repair (13Ajimura M. Leem S.H. Ogawa H. Genetics. 1993; 133: 51-66Crossref PubMed Google Scholar). The purified MRE11 protein (14Petrini J.H. Walsh M.E. DiMare C. Chen X.N. Korenberg J.R. Weaver D.T. Genomics. 1995; 29: 80-86Crossref PubMed Scopus (116) Google Scholar, 15Paull T.T. Gellert M. Mol. Cell. 1998; 1: 969-979Abstract Full Text Full Text PDF PubMed Scopus (701) Google Scholar) and a protein complex containing MRE11 contain 3′ → 5′ exonuclease activities (16Trujillo K.M. Yuan S.S. Lee E.Y. Sung P. J. Biol. Chem. 1998; 273: 21447-21450Abstract Full Text Full Text PDF PubMed Scopus (325) Google Scholar). The TREX1 and TREX2 proteins are relatively small dimeric proteins that contain potent 3′ → 5′ exonucleases (17Mazur D.J. Perrino F.W. J. Biol. Chem. 1999; 274: 19655-19660Abstract Full Text Full Text PDF PubMed Scopus (204) Google Scholar). 1Mazur, D. J., and Perrino, F. W. (2001) J. Biol. Chem., papers in press 10.1074/jbc.M100623200. The presence of 3′ excision activities in this apparently diverse collection of proteins, and likely others, probably reflects the multiple pathways present in human cells requiring the modification of DNA 3′ termini. However, insufficient information is currently available to understand the molecular pathways in which the different 3′ → 5′ exonucleases function. Some insights into the catalytic requirements for 3′ → 5′ exonucleases have been gleaned from protein structure and mutagenesis studies and from protein sequence analysis. The proofreading exonuclease domains of the Escherichia coli DNA polymerase I large fragment and the bacteriophage T4 DNA polymerase have nearly identical folding patterns despite minimal overall sequence identity (18Freemont P.S. Friedman J.M. Beese L.S. Sanderson M.R. Steitz T.A. Proc. Natl. Acad. Sci. U. S. A. 1988; 85: 8924-8928Crossref PubMed Scopus (328) Google Scholar, 19Beese L.S. Steitz T.A. EMBO J. 1991; 10: 25-33Crossref PubMed Scopus (920) Google Scholar, 20Wang J., Yu, P. Lin T.C. Konigsberg W.H. Steitz T.A. Biochemistry. 1996; 35: 8110-8119Crossref PubMed Scopus (105) Google Scholar). Mutagenesis studies identify critical amino acids in three conserved motifs, Exo2 I, Exo II, and Exo III, that are positioned to coordinate two metal ions at the active site (21Derbyshire V. Grindley N.D.F. Joyce C.M. EMBO J. 1991; 10: 17-24Crossref PubMed Scopus Google Scholar, S. E. Konigsberg W.H. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: PubMed Scopus Google Scholar, J. Biol. Chem. 1993; 268: Full Text PDF PubMed Google Scholar). The proofreading exonucleases of the mammalian DNA polymerases also contain these three Exo A. L. J.M. Martin M. Cell. Full Text PDF PubMed Scopus Google Scholar). have been to identify proteins that contain 3′ excision activity M.P. Nucleic Acids Res. 1993; PubMed Scopus Google Scholar, Nucleic Acids Res. PubMed Scopus Google Scholar). This revealed the conserved exonuclease in the WRN protein P. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar, A. Nucleic Acids Res. PubMed Scopus Google Scholar), and the 3′ → 5′ exonuclease activity in this protein (4Shen J.C. Gray M.D. Oshima J. Ashwini S.K. Fry M. Loeb L.A. J. Biol. Chem. 1998; 273: 34139-34144Abstract Full Text Full Text PDF PubMed Scopus (221) Google Scholar, S. B. Gray M.D. Oshima J. J. 1998; PubMed Scopus Google Scholar). The TREX sequences contain the Exo I and Exo and a in the Exo Exo The Exo is the presence of the P. 1995; PubMed Scopus Google Scholar, D. S. Nucleic Acids Res. PubMed Scopus Google Scholar, Nucleic Acids Res. 1998; 26: PubMed Scopus Google Scholar) and is in the of exonucleases M.P. Nucleic Acids Res. 1993; PubMed Scopus Google Scholar, A. Nucleic Acids Res. PubMed Scopus Google Scholar, J. Mol. 1998; PubMed Scopus Google Scholar). The Exo in the TREX proteins that these mammalian exonucleases to the of DNA polymerase III, exonuclease I, and the exonuclease M. J. Biol. Chem. 1999; 274: Full Text Full Text PDF PubMed Scopus Google Scholar). The of 3′ → 5′ exonuclease activities in proteins from human genes that a variety of from the proofreading exonucleases are The multifunctional Escherichia coli exonuclease a potent 3′ excision and the structure of this protein is to the human 1995; PubMed Scopus Google Scholar, S. de E. P.S. EMBO J. 16: PubMed Scopus Google Scholar). in these enzymes indicate a catalytic a metal The 3′ excision activity of the protein is relatively and to and reaction III, M. B. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar) the structure of the 3′ III, B. M. 1999; PubMed Scopus Google Scholar, K.M. M. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google Scholar). The structure of the p53 protein is to the exonuclease and proteins S. Science. 1994; PubMed Scopus Google Scholar), and p53 protein is to contain 3′ → 5′ exonuclease activity (7Mummenbrauer T. Janus F. Müller B. Wiesmüller L. Deppert W. Grosse F. Cell. 1996; 85: 1089-1099Abstract Full Text Full Text PDF PubMed Scopus (246) Google Scholar). The and hRAD9 proteins contain 5′ exonuclease sequence and have insights into the catalytic mechanisms of these proteins C. M.P. Nucleic Acids Res. 2000; PubMed Scopus Google Scholar). studies to identify the of human genes 3′ → 5′ exonucleases. The for TREX1 the 3′ exonuclease activity in from mammalian cells. 3′ exonuclease activity was in and T. T. G.M. J. Biol. Chem. Full Text PDF PubMed Google Scholar). the human and cDNAs 3′ → 5′ exonucleases identified generated from the purified (17Mazur D.J. Perrino F.W. J. Biol. Chem. 1999; 274: 19655-19660Abstract Full Text Full Text PDF PubMed Scopus (204) Google Scholar) and M. P. D.J. J. T. EMBO J. 1999; 18: PubMed Scopus (148) Google Scholar) of a to the for the of to The TREX a for the two 3′ exonuclease genes and is in The of these genes have the of and The TREX reflects the activity and likely three exonucleases. was in data base using the TREX1 cDNA a sequence (17Mazur D.J. Perrino F.W. J. Biol. Chem. 1999; 274: 19655-19660Abstract Full Text Full Text PDF PubMed Scopus (204) Google Scholar). have expression from TREX genes using a and investigated in the 5′-flanking regions of these genes. The human and TREX1 proteins are amino acids in and previously (17Mazur D.J. Perrino F.W. J. Biol. Chem. 1999; 274: 19655-19660Abstract Full Text Full Text PDF PubMed Scopus (204) Google Scholar, M. P. D.J. J. T. EMBO J. 1999; 18: PubMed Scopus (148) Google Scholar). Our expression of TREX1 and the evidence for the expression of TREX2 in human cells. Novel cDNAs containing the TREX1 and TREX2 ORFs have been identified that contain exons 18 for TREX1 and 25 for The of the TREX genes are in this in the DNA of the and are in I. The genomic DNA was from The genomic DNA was a from P. of The human genomic DNA was the from for amplification reverse reverse in a amplification of TREX1 from genomic the of genomic and of the and reverse The TREX2 also to for to DNA polymerase at The for at for for and for The from the using and using a DNA cDNA from was to recover the 5′-flanking regions of TREX1 and TREX2 The to the using the and the and pairs in and The TREX2 also The and from the using in three of the in the from into the and was from cells of a acute myeloblastic leukemia and PubMed Scopus Google Scholar). The was I and purified using a The was to of for at and at The was reverse for at to generate The using cDNA and the cDNA for genomic The pairs for the of cDNA are in and The for the was of reverse and the for the was a of the The from the from the and was for TREX1 and TREX2 expression using the human cDNAs and the in the of TREX2 transcripts and expression in human The pathways from site to site site and the three TREX2 transcripts are The and are using the pairs and of the generated using reverse the presence of all three TREX2 transcripts. contains DNA from human tissues was to using the and of the the presence of the TREX2 in all Potential exons in the genomic DNA of the 5′-flanking regions of the TREX1 and TREX2 ORFs identified using the C. S. J. Mol. Biol. 268: PubMed Scopus Google Scholar). The for amplification of novel TREX1 and TREX2 cDNAs for genomic The pairs in the two amplification are in the and in and The from the from the and transcripts. The genomic sequence of a 25 region of human containing the TREX2 ORF is The of the exons in the genomic sequence and in the sequence are human from the GenBank™ data base is the The are The novel TREX2 cDNAs in this using the and The of transcription initiation sites −753 and −623 sites and are The sequences generated from a purified mammalian 3′ → 5′ exonuclease identified the human TREX1 cDNA from in the GenBank™ data base in two studies (17Mazur D.J. Perrino F.W. J. Biol. Chem. 1999; 274: 19655-19660Abstract Full Text Full Text PDF PubMed Scopus (204) Google Scholar, M. P. D.J. J. T. EMBO J. 1999; 18: PubMed Scopus (148) Google Scholar). the TREX1 sequence in a of the GenBank™ data base to identify TREX1 no. and the human of these TREX1 in the 5′-flanking regions These sequence the in this of the TREX1 cDNAs and the TREX1 genomic sequence in the human The genomic DNA sequences the and human TREX1 genes to the ORF structure of this studies of human and TREX1 cDNA sequences identified a positioned the 5′ of the TREX1 ORFs (17Mazur D.J. Perrino F.W. J. Biol. Chem. 1999; 274: 19655-19660Abstract Full Text Full Text PDF PubMed Scopus (204) Google Scholar). The proteins produced from the human and TREX1 cDNAs using this a start generated active 3′ → 5′ exonucleases. However, TREX1 no. contain a nucleotides upstream the that the in the TREX1 ORF been identify the for genomic positioned at the 5′ of the and human TREX1 ORFs using and the sequences of these pairs in these from the TREX1 in and the available sequences in the GenBank™ data base The of the genomic DNA from the and of the the genomic sequences identified consensus and sequences and that the 5′-flanking regions of the TREX1 transcripts The amino sequences from the genomic DNA sequences and using to the identity at the 5′ ends of the TREX1 ORFs The that the is the conserved in all three mammalian that this is the of the TREX1 ORF. sequence identity is to the the genomic sequences for and genomic DNA contain at and providing for the of the for TREX1 The human genomic sequence two at and The of these is currently The human and TREX1 sequences at the are identical to the consensus sequence M. Nucleic Acids Res. PubMed Scopus Google Scholar), and the sequence at a of genomic DNA have the ORF of TREX1 in the and human The human and TREX1 ORFs indicate a region of amino and the TREX1 ORF is amino acids in TREX a protein of amino acids and contains two The the mammalian and genes is apparent using the Nucleic Acids Res. 2000; PubMed Scopus Google Scholar). The of these genes into the of of proteins the E. coli DNA polymerase the these proteins is likely to the of nucleotides from DNA 3′ the these the and the are The 5′-flanking region of TREX1 cDNAs was using a was using cDNA the reverse and and the cDNA clones from to in and these sequences the TREX1 genomic sequence identify genomic sequences in the 5′-flanking region of TREX1 that transcription initiation the sequence Neural Network Promoter Prediction was promoters positioned −140 and −650 from the TREX1 ORF are identified The 5′ ends of TREX1 cDNAs the genomic sequence at transcription initiation at the −650 consensus putative promoter of the cDNAs and at transcription initiation at the −140 the −650 putative promoter In the 5′ of another cDNA was positioned 5′ to promoter alternative promoters are present in the region of of the cDNAs and at consensus and sequences that been previously identified in human providing for a modification of the 5′-flanking region of TREX1 transcripts. The TREX1 cDNA sequences in the the 5′-flanking regions of TREX1 available in the GenBank™ data These sequences the presence of one sequence and two sequences in to the TREX1 two pathways for of the TREX1 transcripts. was that from the site to site generate a the TREX1 to site generate a that TREX1 sequence to encode active TREX1 protein is that these TREX1 transcripts a for of TREX1 of the was to the of the three TREX1 transcripts using reverse from cells. The TREX1 cDNAs using the TREX1 and and the 5′-flanking region and The three TREX1 transcripts of the of the the identity of the the of the TREX1 The and from amplification of the two TREX1 transcripts. The is the generated from the site to site positioned base pairs 5′ to the These data indicate that the TREX1 transcripts in at the −650 putative are that a from the 5′-flanking region of the TREX1 transcripts. of and TREX1 in the data base a to conserved sites positioned at in and in to the of this mammalian The site to site is the for of TREX1 transcripts in human the of TREX1 expression in human a was using the and and reverse from different tissues The three generated from TREX1 transcripts are in all tissues In the of the three are in the the in all Some in the of the are in expression These data that TREX1 expression in and cells is that in and to these in expression of TREX1 is from these In from this active 3′ → 5′ exonuclease was generated from a identified in the GenBank™ data base (17Mazur D.J. Perrino F.W. J. Biol. Chem. 1999; 274: 19655-19660Abstract Full Text Full Text PDF PubMed Scopus (204) Google Scholar). from have been in the GenBank™ data was to identify human TREX2 cDNAs and to the 5′-flanking region of these cDNAs using a was using cDNA reverse and and the cDNA clones in from to and the TREX2 genomic sequence The genomic sequence in the 5′-flanking region of TREX2 was using the to identify transcription initiation promoters positioned −623 and −753 from the TREX2 ORF identified The 5′ ends of TREX2 cDNAs the genomic sequence at transcription initiation at one of these consensus putative promoter The 5′ of another cDNA was positioned 5′ to alternative promoters are present upstream in the 5′-flanking region of The genomic DNA sequence in the 5′-flanking region of the human TREX2 was for and sequence and two sequences the for a for TREX2 transcripts to that for TREX1 transcripts the TREX1 two pathways are for of the TREX2 transcripts. However, of a TREX1 from the site to site to site a TREX2 the ORF. was using the TREX2 and and the 5′-flanking region and cDNA to recover TREX2 The three TREX2 transcripts are of the of the the identity of the the of the TREX2 The and from amplification of the two TREX2 transcripts. The of the likely reflects the of the TREX2 transcripts in cells the TREX2 the These results the evidence for expression of the TREX2 in human cells and identify that a a from the region of the TREX2 transcripts. The sequence of the in the 5′-flanking region from the site to site to remove a of this mammalian was to the expression for TREX2 in human cells. The of TREX2 expression was in a of using the and and reverse from different tissues The generated from the TREX2 was in all tissues The TREX2 transcripts in this that the of the TREX2 is the in all tissues The of the generated from and cDNA from and some in expression of TREX2 in human cells. The of TREX1 and TREX2 transcripts from all human tissues these proteins in functions the DNA repair processes to the of the human genome. the human TREX1 ORF to M. P. D.J. J. T. EMBO J. 1999; 18: PubMed Scopus (148) Google Scholar). However, genomic sequence in this region a of the sequence the TREX1 ORF. the TREX1 cDNA the sequence in a of the GenBank™ genomic sequences data base to identify the human The presence of TREX1 this genomic DNA fragment the of TREX1 to the The DNA is a sequence of of The of the containing the TREX1 ORF was using and in the GenBank™ data base and a genomic sequence in the GenBank™ data base the two and 5′ to the TREX1 I. The protein sequence of the of the the human in the data of the data base using the the sequence a that the 5′ a of the data base using the a sequence no. that indicate the of the V. DNA containing 25 of genomic DNA have been to the genomic sequence positioned 5′ to the TREX1 ORF. the that in analysis. was to identify novel TREX1 cDNAs transcription initiation sites positioned 18 5′ to the TREX1 ORF. The exons this region identified using the C. S. J. Mol. Biol. 268: PubMed Scopus Google Scholar). a protein of amino acids containing exons the TREX1 ORF the 3′ is this sequence have been However, human containing exons exons and exons and have been identified was to identify novel TREX1 cDNAs that contain the sequences in the 5′-flanking region a was using the TREX1 and and the 5′-flanking region and cDNA to cDNAs containing exons from and The sequences the presence of exons in these TREX1 cDNAs the sequences indicate the from transcripts from to using the cDNA was from to using the TREX1 and the 5′-flanking region and to cDNAs containing exons the 5′-flanking region and to cDNAs containing exons TREX1 cDNAs identified from these The cDNA contains the exons and two exons and The cDNA contains the exons and and The TREX1 cDNAs identified in these the identified exons and generate these transcripts synthesis the and the TREX1 ORF to the consensus These data indicate a complex of transcription initiation and for the mammalian TREX1 and the TREX1 ORF and exons identified in the 5′-flanking transcription initiation 5′ to the was using cDNA the and from and sequences and the cDNA clones and these sequences the TREX1 genomic sequence The 5′ ends of these cDNAs are of the initiation in of this sequence using the a transcription initiation site positioned 5′ to the initiation in This transcription initiation site is 5′ to the TREX1 ORF In a was that the TREX2 ORF was of a ORF in a genomic from M. P. D.J. J. T. EMBO J. 1999; 18: PubMed Scopus (148) Google Scholar). This a protein of amino acids containing exons the TREX2 ORF the 3′ are that to this However, a human containing of the exons in the ORF been identified was to identify novel TREX2 cDNAs that contain exons in the 5′-flanking region of the TREX2 ORF. was using the TREX2 and and the 5′-flanking region and cDNA to cDNAs containing exons in the 5′-flanking from and The cDNAs the presence of transcripts containing exons that the 5′-flanking region of TREX2 of the exons and are present in the TREX2 and in the are present in of the exons are in the TREX2 contains multiple in all three reading that the ORF in these TREX2 In TREX2 cDNA is present in of these TREX2 transcripts synthesis the and the TREX2 ORF to the consensus of these novel TREX2 cDNAs the that transcription initiation in the 5′-flanking region generate transcripts that to contain a ORF the TREX2 sequence the 3′ also transcription initiation 5′ to the was using cDNA from sequences and and the cDNA clones and these sequences the TREX2 genomic sequence The 5′ ends of these cDNAs are the initiation in of this sequence using the a transcription initiation site positioned upstream from the initiation in This transcription initiation site is 25 5′ to the TREX2 ORF. The of novel TREX1 and TREX2 transcripts containing a structure a complex of expression for these genes. The the ORFs in the 5′-flanking regions and the TREX1 and TREX2 ORFs are the to the TREX ORFs the of the been In have that the TREX1 and TREX2 genes encode mammalian 3′ → 5′ exonucleases that are expressed in all human tissues are a of in the and in the expression patterns of the TREX genes. The genomic sequence the TREX1 protein is in a ORF. The TREX2 protein is also in a ORF. TREX1 and transcripts are of the exonuclease and sequences are from the region two sites of transcription initiation are identified at 18 5′ to the TREX1 ORF and 25 5′ to the TREX2 ORF transcripts that contain the TREX ORF and a upstream ORF of function. The of TREX1 and TREX2 transcripts in all human cells the expression of these genes and a for these 3′ exonucleases in DNA repair pathways in human cells. for this and and for analysis.