Fig. 2.

Switch behavior of pCF10-carrying donor cells in response to pheromone induction. (A–D) qRT-PCR analysis of the dynamics of expression of conjugation determinants encoded by the prgQ operon in response to pheromone induction. mRNA was purified from cultures of pCF10-containing donors exposed to different concentrations of C for various periods. Transcript levels are shown for Q_L (A), prgB (B), pcfC (C), and pcfG (D), normalized relative to a transcript from a constitutive chromosomal gene, gyrB, and to time t = 0. Data shown in A–D are averages of three independent experiments (error bars are SDs from mean values). (E–G) Mathematical modeling of the pCF10-encoded pheromone response. (E) The steady-state response of Q_L (normalized to the off state) to induction demonstrates a characteristic bistable switch behavior. The bistable region shifts to a higher level of cCF10 with increasing donor density, accompanied by an increasing threshold level of cCF10 to turn on the conjugative genes. (F and G) Simulated dynamic response of Q_L transcript level (normalized to the off state at time t = 0) to C induction at various donor cell densities and at two different C concentrations: 1 ng/mL (F), and 10 ng/mL (G). At low donor density, Q_L is sustained at a high level over a longer period. At a high donor density, it decreases rapidly after a short period of high-level expression. At a high level of induction (10 ng/mL) corresponding to high a level of recipient cell density, Q_L stays high for a longer period.