Organization of the DNA inversion system of P1. The two cix recombination sites flanking the invertible C-segment, and the enhancer sequence sis, located within the cin gene, are shown as black boxes. Two promoters, Ps, expressing the tailfiber operon and Pcin, expressing the cin gene are shown as filled triangles. Genes are shown as open boxes with arrowheads indicating the direction of transcription. In the depicted orientation the three tailfiber proteins R, S and U are produced, while a single inversion event would fuse Sc to Sv', resulting in the expression of the three proteins R, S' and U'.