Figure 4.

cAMP wave propagation. (A) Wild-type and icmA⁻ cells were plated side by side at the same density on nonnutrient agar, separated by a plastic divider (center of each image). Waves propagate normal to the convex surface of each wave. The left half of each frame is wild type; the right half is icmA⁻. The dark bands correspond to peak cAMP concentration, and the lighter bands correspond to cells that are either refractory to cAMP (trailing edge of the expanding wave) or excitable (leading edge). Wild-type cells form typical target and spiral patterns whose frequency increases with time. Signaling in icmA⁻ cells is delayed by ∼3 h, the wave frequency does not increase with time, and the waves die out before there has been substantial cell movement. By 4:45, wild-type cells have begun to aggregate and wave propagation has died down. Times are in hours and minutes after washed cells were plated on agar. (B) icmA⁻ cells respond more vigorously to cAMP stimulation than wild-type cells. Cells starved for 1 h were then pulsed for 5 h with nanomolar cAMP, stimulated with 10 μM 2′-deoxy-cAMP, and sampled at frequent intervals. Total cAMP was then measured by a protein kinase assay. Each point is the average of four independent reactions. The error bars are ±SD. In each reaction, cAMP levels were measured with 4 × 10⁶ cells.