THE RASPBERRY LOCUS OF DROSOPHILA-MELANOGASTER INCLUDES AN INOSINE MONOPHOSPHATE DEHYDROGENASE LIKE CODING SEQUENCE

DNA from the raspberry gene of Drosophila melanogaster has been cloned through P-element tagging of a dysgenically induced lethal allele. A transcribed DNA segment adjacent to the P insert encodes an amino acid sequence that is similar to known inosine monophosphate dehydrogenase (IMPDH) sequences. Further dysgenically induced lethals and all four known spontaneous ras eye-colour mutations have changes in the DNA eit her within or just upstream from the transcribed region. Identificatio n of IMPDH as a probable product of the ms gene is compatible with the finding of an allele that requires dietary guanosine (GR), since the enzyme mediates the first of two biosynthetic steps leading from inosi ne monophosphate (IMP) to guanosine monophosphate (GMP). However, othe r auxotrophic mutants at the locus remain unexplained by the finding. The results further suggest that GMP synthesis from IMP is an essentia l process, despite the capacity of the fly for salvage synthesis of GM P from GR. Consideration of the phenotypes associated with mutants at the ms locus suggests that IMPDH activity is regulated in a tissue-spe cific manner.