FIG. 1.

The P element construct serves as a target for Gal4, T7RNAP, and FLP. (A) Diagram of the P element. The initial P insertions contain two cassettes, each flanked by directly oriented FRT sequences. The left cassette contains the rosy transformation marker, and a “homing fragment,” which targets P elements to the BX-C chromosome region. The right cassette contains two tandem T7 promoters and a Gal4 activatable UAS-LacZ reporter. (B) Trimmed-down insertions, which lack the homing/rosy ⁺ cassette, were recovered as rosy⁻ LacZ⁺ individuals following FLP induction in the germ line. The T7 promoters are poised to transcribe the genomic DNA flanking the 3′ P end. (C) Expression of FLP recombinase in a whole embryo results in excision of the T7 promoter/UAS-LacZ cassette from the chromosome, resulting in formation of a 4.9-kb circular episome in somatic cells. The T7 promoters are then poised to produce antisense copies of LacZ RNA.