Consider a transcription regulatory protein (A) that can bind to the promoter (p) of a target gene. After reaching an initial steady state, the concentration of A is suddenly increased tenfold. The following graph shows the formation of A:p over time after this change. What is the slope of the curve at time zero?
A) 2 kₒff
B) 2 kₒn
C) 5 kₒff
D) 5 kₒn
E) 9 kₒff

Question

Consider a transcription regulatory protein (A) that can bind to the promoter (p) of a target gene. After reaching an initial steady state, the concentration of A is suddenly increased tenfold. The following graph shows the formation of A:p over time after this change. What is the slope of the curve at time zero? 
A) 2 kₒff 
B) 2 kₒn 
C) 5 kₒff 
D) 5 kₒn 
E) 9 kₒff

Quizplus · Accepted Answer

The slope of the curve (i.e. d[A:p]/dt) at any time is equal to the rate of complex formation minus the rate of complex dissociation. At the initial steady state (before the concentration jump), the two terms are equal. The rate of complex formation is kₒn [A][p] and is suddenly increased tenfold at t = 0. The rate of dissociation is kₒff [A:p] (or just kₒff since [A:p]t₌₀ is assigned a value of 1) and is unchanged at t = 0. Therefore, the slope increases from 0 to 9 kₒff at time zero. Please refer to the equations in Figure 8-73.

Consider a Transcription Regulatory Protein (A) That Can Bind to the Promoter