The sugarless mutation affects the expression of the white eye color gene in Drosophila melanogaster

Investigation of a modifier locus displaying a darker eye phenotype in whit e-apricot flies led to the isolation of the gene encoding UDP-glucose dehyd rogenase (UDPGDH). The P-element insertion 1(3)05007 occurs upstream of the transcription start site of the sugarless (sgl) gene and greatly reduces i ts transcription at various developmental stages. A single abundant sgl tra nscript shows a ubiquitous distribution and encodes a 53-kDa protein which is 64% identical in sequence to bovine UDP-glucose dehydrogenase. Overexpre ssion of sgl in E. coli resulted in synthesis of a protein with high levels of UDPGDH activity. Expression of three genes that participate in pigment deposition, white, scarlet and brown, was significantly affected in populat ions segregating for sgl, suggesting that it is the decrease in UDPGDH leve l that produces the modifying effect observed. In addition, genetic effects on white-apricot were observed in sgl-wingless and sgl-hedgehog double mut ants. Recent data have indicated an effect of UDPGDH on cell surface glycos aminoglycans (GAGs) which modulate the activity of growth factors, and in p articular wingless signaling. Our results suggest that the levels of GAGs a re rate limiting for cell-cell signaling pathways which mediate changes in gene expression.