Figure 1. Organization of the pAD1 par locus and the Fst toxin. Converging promoters (black arrowheads labeled P) transcribe the toxin-encoding RNA I (red shaded arrow below line) and the antitoxin RNA II (dark green arrow above line) toward a bi-directional intrinsic transcriptional terminator (converging green arrows). The RNAs are transcribed across direct repeats DRa (pink arrows) and DRb (blue arrows) at which interaction occurs, suppressing translation of the Fst_pAD1 coding sequence (dark red box on DNA and RNA). The amino acid sequence of the Fst toxin is shown using standard single letter amino acid designations. The essential, conserved hydrophobic domain is shown in bold red print. This forms part of a transmembrane domain in the recently published structure of Fst.²² The two blue underlined amino acids at the N-terminus must be charged to retain toxin function. The non-essential C-terminal tail is shown in green italics.