Mutation of a novel ABC transporter gene is responsible for the failure to incorporate uric acid in the epidermis of ok mutants of the silkworm, Bombyx mori

• Based on positional cloning and RNA interference, the gene responsible for the ok mutants was determined to be Bm-ok.
• Mutations in the genomic sequence of Bm-ok generated abnormal transcripts in the two independent ok mutants.
• Embryonic RNA interference for Bm-ok elicited a translucent larval phenotype similar to that of the ok mutants.
• Bm-ok and its orthologs in insects constitute a new subfamily, which is distinct from known ABC transporters.

ok mutants of the silkworm, Bombyx mori, exhibit highly translucent larval skin resulting from the inability to incorporate uric acid into the epidermal cells. Here we report the identification of a gene responsible for the ok mutation using positional cloning and RNAi experiments. In two independent ok mutant strains, we found a 49-bp deletion and a 233-bp duplication, respectively, in mRNAs of a novel gene, Bm-ok, which encodes a half-type ABC transporter, each of which results in translation of a truncated protein in each mutant. Although the Bm-ok sequence was homologous to well-known transporter genes, white, scarlet, and brown in Drosophila, the discovery of novel orthologs in the genomes of lepidopteran, hymenopteran, and hemipteran insects identifies it as a member of a new distinct subfamily of transporters. Embryonic RNAi of Bm-ok demonstrated that repression of Bm-ok causes a translucent phenotype in the first-instar silkworm larva. We discuss the possibility that Bm-ok forms a heterodimer with another half-type ABC transporter, Bmwh3, and acts as a uric acid transporter in the silkworm epidermis.

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