ANALYSIS OF A MOUSE GENE ENCODING 3 STEPS OF PURINE SYNTHESIS REVEALSUSE OF AN INTRONIC POLYADENYLATION SIGNAL WITHOUT ALTERNATIVE EXON USAGE

A single mouse genomic locus encodes proteins catalyzing three steps o f purine synthesis, glycinamide ribonucleotide synthetase (GARS), amin oimidazole ribonucleotide synthetase (AIRS), and glycinamide ribo nucl eotide formyltransferase (GART). This gene has 22 exons and spans 28 k ilobases. The existence of a second genetic locus and closely related pseudogenes was ruled out by Southern analysis, Mouse tissues express two related classes of messages encoded by this single locus: a trifun ctional GARS-AIRS-GART mRNA and a monofunctional GARS mRNA. These tran scripts used the same set of multiple transcriptional start sites, and both used the same first 10 exons, CCAAT and TATA elements were not f ound for this locus. Exon 11, which represented the last coding sequen ce of the GARS domain, was differentially utilized for the two message s, The trifunctional mRNA was generated by splicing exon 11 to exon 12 , the first coding sequence for the AIRS domain with subsequent use of a polyadenylation signal at the end of exon 22. Genomic sequence corr esponding to the 3'-UTR of the monofunctional GARS mRNA was contiguous with exon 11, so that the smaller message arose from the recognition of one of the multiple polyadenylation signals present within the intr on between exons 11 and 12. Hence, polyadenylation of the primary tran script at a position corresponding to an intron of the genomic locus w as responsible for the generation of the monofunctional GARS class of mRNAs. This utilization of an intronic polyadenylation site without al ternative exon usage is comparable to the mechanism whereby both secre ted and membrane bound forms of the immunoglobulin mu heavy chain are made from a single genetic locus.