Gene within a gene: nested Drosophila genes encode unrelated proteins on opposite DNA strands.

A pupal cuticle protein gene has been found within an intron of a Drosophila gene that encodes three purine pathway enzymatic activities. The intronic gene is encoded on the DNA strand opposite the purine pathway gene and is itself interrupted by an intron. Whereas the purine pathway gene is active throughout development, the intronic cuticle protein gene is expressed primarily over a 3 hr period in the abdominal epidermal cells of prepupae that secrete the pupal cuticle. Therefore, a housekeeping gene and a developmentally regulated gene function in a nested arrangement.