Plasticity of the yin element over time. (a) PCR amplification of the yin element integrated into the chromosome, the intact yabG gene, and the circular yin element in C. botulinum ST7B and V73 grown in TPGY medium. DNA-free water was used as a negative control. The amplicons (black bars) are shown near each DNA gel. Legend: Neg., negative control. (b) Quantitative PCR results to determine the relative proportion of restored yabG gene in C. botulinum ST7B and V73 over time. yabG primers span across the att recombination sites, whereas the control primers amplify a region elsewhere in the yabG gene. Error bars represent the standard deviations based on four technical replicates. Legend: pink curve, strain V73; blue curve, strain ST7B.