Structure and Expression of Wild-Type and Suppressible Alleles of the Drosophila purple Gene

Abstract

Viable mutant alleles of purple (pr), such as prbw, exhibit mutant eye colors. This reflects low 6-pyruvoyl tetrahydropterin (PTP) synthase activity required for pigment synthesis. PTP synthase is also required for synthesis of the enzyme cofactor biopterin; presumably this is why some pr alleles are lethal. The prbw eye color phenotype is suppressed by suppressor of sable [su(s)] mutations. The pr gene was cloned to explore the mechanism of this suppression. pr produces two PTP synthase mRNAs: one constitutively from a distal promoter and one in late pupae and young adult heads from a proximal promoter. The latter presumably supports eye pigment synthesis. The prbw allele has a 412 retrotransposon in an intron spliced from both mRNAs. However, the head-specific mRNA is reduced >10-fold in prbw and is restored by a su(s) mutation, while the constitutive transcript is barely affected. The Su(s) protein probably alters processing of RNA containing 412. Because the intron containing 412 is the first in the head-specific mRNA and the second in the constitutive mRNA, binding of splicing machinery to nascent transcripts before the 412 insertion is transcribed may preclude the effects of Su(s) protein.

Biosynthesis of Pteridines in Insects: A Review

“…This compound is converted to 6-pyruvoyl-5,6,7,8-tet dropterin by the action of the enzyme pyruvoyl-tetrahydropterin synthase, for known as sepiapterin synthase A [12][13][14][15]. The purple (pr) gene, located at 38B3 on th ond chromosome in D. melanogaster, is the structural gene of this enzyme [16]. GTP is converted to 7,8-dihydroneopterin triphosphate by the action of the enzyme GTP cyclohydrolase I [9,10].…”