Fig. 4.

Thymine requirements of thyA and thyA yjjG strains with different dgt expression levels determined on thymine gradient plates. A. Suspensions of the indicated cultures were streaked on minimal-medium plates (Min) or minimal-medium plates supplemented with 0.5% casamino acids (Min + CAA). The plates contained a gradient from top to bottom of decreasing thymine concentration from 40 to 0 µg/ml. B. Histogram of streak lengths (extent of growth) for each of the strains: black bars, (Min); dashed bars, (Min+CAA). A total of ten plates was used for standard error calculation. The modest difference between the dgt and dgt⁺ strains on the Min plate (top left) (0–40 µg/ml gradient) contrasts with the very obvious difference observed between the two strains on the (0–20 µg/ml) gradient plate (Fig. 2A). This phenomenon, observed repeatedly, is likely related to the difference in steepness of the gradients, where a steeper gradient is predicted to diminish the apparent difference in growth (i.e., streak length).