FIG 1.

E. coli's single-cell response to the sugars l-arabinose and d-xylose. (A) A population of E. coli cells exposed to intermediate amounts of both sugars diverges into four subpopulations: uninduced cells, cells with the l-arabinose utilization pathway induced, cells with the d-xylose utilization pathway induced, and cells with both pathways induced. The fraction of each subpopulation depends on the concentrations of l-arabinose and d-xylose. (B) The two utilization pathways repress each other. In the presence of l-arabinose, AraC represses the expression of the d-xylose catabolic genes. In the presence of d-xylose, XylR represses the expression of araC.