Multiple mRNAs from the Punch locus of Drosophila melanogaster encode isoforms of GTP cyclohydrolase I with distinct N-terminal domains.

The GTP cyclohydrolase I gene of Drosophila melanogaster, the Punch locus, encodes alternative transcripts of 1.7 and 1.75 kilobases (kb). These transcripts are differentially expressed throughout Drosophila development. cDNA clones representing these transcripts, corresponding genomic regions and polymerase chain reaction-amplified primer extensions of the 5' ends of the RNAs were sequenced. Both RNAs contain five exons and derive from primary transcripts that extend over approximately 8 and 4 kb for the 1.7- and 1.75-kb RNAs, respectively. Their 5'-most exons are unique and spliced onto common 3' exons. The cDNAs each contain a single long open reading frame, which can be translated into a polypeptide of 273 amino acids for the 1.7-kb mRNA and 308 amino acids for the 1.75-kb mRNA. The unique exons confer distinct N-terminal domains to each predicted protein. Sequence comparisons reveal that the Drosophila GTP cyclohydrolase isoforms encoded by the multiple transcripts are highly similar to GTP cyclohydrolases from humans, rodents, and bacteria, with one significant exception. The N-terminal domains encoded by the transcript-specific 5' exons cannot be aligned with the N termini of any other GTP cyclohydrolases. These domains are predicted to confer distinct physical characteristics to the alternate isoforms, and it is hypothesized that they aid in regulating the expression of the enzyme in diverse cellular environments.