A deleted portion of one of the two xanthine dehydrogenase genes causes translucent larval skin in the oq mutant of the silkworm (Bombyx mori).

A silkworm mutant, oq, has translucent larval skin because it is deficient in xanthine dehydrogenase (XDH) activity and is unable to synthesize uric acid, which is normally accumulated in the larval epidermis and makes the skin white and opaque. Two XDH bands were found in zymograms of the silkworm fat body: an intense band (XDHalpha) and a faint one (XDHbeta). The oq mutant lacks only XDHalpha, which seemed to be the major source of XDH activity in the fat body. An 8-bp deletion found in BmXDH1, a silkworm XDH gene, generates a premature stop codon. The resulting truncated BmXDH1 protein lacks three molybdenum cofactor-binding domains necessary for enzyme activity. BmXDH2, the other XDH gene, does not show any apparent deficiencies. BmXDH1 expressed in yeast cells yielded an activity band with the same mobility as that of XDHalpha in zymograms. BmXDH1 of the oq mutant did not yield active XDH in yeast, while the activity was restored by filling in the deleted sequence. These results showed that BmXDH1 deletion in the oq mutant is responsible for the absence of significant XDH activity, resulting in the translucent larval skin of the mutant phenotype.