Molecular characterization of a nonsuppressible allele (prC4) of the Drosophila purple gene.

: Purple gene encodes 6-pyruvoyl tetrahydropterin synthase (PTP synthase) in Drosophila. The enzyme PTP synthase catalyzes the conversion of dihydroneopterin triphosphate (H2-NTP) to 6-pyruvoyl tetrahydropterin (PTP), an important intermediate for pterin compounds. The extreme purple mutant, prC4, shows a very low activity of PTP synthase. The mutant purple gene has been cloned by screening with the subgenomic library of prC4. The size and expression level of PTP synthase gene transcripts in prC4 were almost the same as those of the wild type. The genomic DNA was also examined in the purple region by Southern blot analysis, but no changes in restriction pattern could be detected. Compared with the wild type PTP synthase sequence, the mutant PTP synthase of prC4 showed three missense mutations: the replacement of alanine 7 by serine (A7S), leucine 9 by phenylalanine (L9P), and aspartic acid 168 by glycine (D168G). Significance of these mutations was discussed in relation to the formation of the oligomeric structure of PTP synthase.