Figure 1. Design Principles of a Fluorescent Reporter of Cell Cycle Speed.

(A) On the basis of the equation in Data S1, intracellular levels of a molecule (M) depend on its half-life and cell cycle length. The ratio of M’s concentration in theoretical cells of different cycling speeds is plotted. When M is short lived (circle, far left), its concentration shows little difference between slow and fast cycling cells. With long-lived M (circle, far right), the concentration difference increases in proportion to the difference in cell cycle lengths.

(B) The fluorescent timer (FT) is short lived as a blue protein and long lived as a red protein.

(C) The average blue/red ratio (BR) of cells expressing FT drops as cell cycle lengthens (see Data S2). Because BR fluctuates within a cell cycle (see Figure S1), the relationship between BR and cell cycle length is best described by a probability distribution. For modeling, cells are assumed to maintain a constant cycling rate. Solid and dashed error bars denote 1 and 2 SDs, respectively.

(D) Anticipated positions of slow and fast asymmetrically cycling cells on a hypothetical plot of blue versus red fluorescence.