FIG. 7.

FLP recombinase is partially blocked by the PcG. RNA in situ hybridizations were used to distinguish T7 promoters in the chromosome or in FLP-induced circles in germ band retracted embryos. (A to E) Control embryos lacking FLP. RNA probes detect transcription of flanking genomic sequences from the T7 promoters in the P insertions. In the absence of FLP, all T7 promoter cassettes remain in the chromosome and are poised to transcribe the flanking genomic DNA. Under these assay conditions, no segmental bias in transcription by T7RNAP is apparent in any of the fly lines. (F to J) Transcription of chromosome sequences by T7RNAP marks cells in which FLP has failed to access its FRT target sites, which flank the T7 promoter/UAS-LacZ cassette. (F) FLP efficiently excises the cassette from the majority of the cells in a control line, in which the cassette is located on the X chromosome. (G to J) In the BX-C insertion lines, FLP fails to excise the cassette in many cells in PcG-repressed segments. The block to circle formation appears to occur one parasegment anterior to the block to Pol II transcription. The posterior-most parasegment in which chromosomal transcription is strong is marked with a bracket. (K to O) Transcription of antisense LacZ RNA by T7RNAP marks cells in which FLP has succeeded in producing a circular episome. (K) Circles are visible in most cells in the control embryo. (L to O) A greater number of circles are visible in non-PcG-repressed segments in the BX-C insertion lines. The anterior-most parasegment in which robust circle formation is visible is marked with a bracket.