Comparative analysis of the nonA region in Drosophila identifies a highly diverged 5 ' gene that may constrain nonA promoter evolution

A genomic fragment from Drosophila virilis that contained all the no-on-tra nsientA (nonA) coding information, plus several kilobases of upstream mater ial, was identified. Comparisons of nonA sequences and the gene non;l-like in D, melanogaster, a processed duplication of nonA, suggest that it arose before the split between D, melanogaster and D. virilis. In both species, a nother gene that lies <350 bp upstream from the nonA transcription starts, and that probably corresponds to the lethal gene l(1)i19, was identified. T his gene encodes a protein that shows similarities to GPI1,which is require d for the biosynthesis of glycosylphosphatidylinositol (GPI), a component f or anchoring eukaryotic proteins to membranes, and so we have named it dGpi 1. The molecular evolution of nonA and dGpi1 sequences show remarkable diff erences, with the latter revealing a level of amino acid divergence that is as high as that of transformer and with extremely low levels of codon bias . Nevertheless, in D. melanogaster hosts, the D. virilis fragment rescues t he lethality associated with a mutation of l(1)i19e, as well as the viabili ty and visual defects produced by deletion of nonA(-). The presence of dGPi 1 sequences so close to nonA appears to hare constrained the evolution of t he nonA promoter.