Fig. 3

Dynamics of ON/OFF sense transcription in the absence (left) and presence (right) of cognate regulatory antisense transcript. Left panel (in the absence of regulatory antisense RNA): at low levels of activating stimuli, OFF state of sense transcription is unstable and noisy, and it is induced stochastically in response to transient fluctuations of the signal. As activating stimuli strengthen, sense transcription slowly increases and gradually reaches maximum levels along a hyperbolic curve (ON). Similarly, return to baseline levels of sense transcription occurs gradually with the waning of the activating stimuli. Right panel (in the presence of regulatory antisense RNA): at low levels of activating stimuli (OFF), the antisense transcript buffers spurious activating stimuli, thus dampening noisy expression of sense transcripts and achieving a stable OFF state. When activating stimuli are sufficiently strong to overcome the threshold set by the antisense RNA, sense transcription is induced and rapidly reaches maximum levels along a sigmoid curve (ON). As activating stimuli weaken below the ultrasensitive threshold, expression of sense transcripts rapidly drops to baseline levels following an inverse sigmoid curve and returns to a stable OFF state