MOUSE CDNAS ENCODING A TRIFUNCTIONAL PROTEIN OF DE-NOVO PURINE SYNTHESIS AND A RELATED SINGLE-DOMAIN GLYCINAMIDE RIBONUCLEOTIDE SYNTHETASE

Three of the enzymatic activities of de novo purine synthesis, glycina mide ribonucleotide synthetase (GARS), aminoimidazole ribonucleotide s ynthetase (AIRS) and glycinamide ribonucleotide formyltransferase (GAR T), can be catalyzed by a single ll0-kDa protein in mouse cells. Weste rn blots using a polyclonal antibody (Ab) to this protein identified t wo species, the trifunctional ll0-kDa protein and a 50-kDa cytosolic p rotein with GARS, but not GART activity. We used Ab and, subsequently, oligodeoxyribonucleotide screens to isolate cDNAs corresponding to th ese two proteins from mouse T-cell cDNA expression libraries. The sequ ence of one class of these cDNAs and the partial sequence of a corresp onding genomic clone defined an open reading frame (ORF) encoding a 10 10-amino-acid (aa) protein, individual domains of which showed high ho mology to each of the monofunctional bacterial GARS, AIRS and GART pro teins, and to each domain of chicken and human trifunctional GARS-AIRS -GARTs. cDNAs corresponding to the smaller protein contained a 1.3-kb ORF with complete identity to the GARS domain of, but with a 3' untran slated region different from, the trifunctional cDNAs. Hence, both cDN As appear to derive from the same gene due to either differential spli cing or use of an intronic polyadenylation signal. The functional requ irement for the expression of both trifunctional protein with GARS act ivity and monofunctional, catalytically active GARS is unknown.