Fig. 5.

Models for C-signal-dependent regulation of fmgE and fmgD. As development proceeds, C-signaling causes the concentration of active FruA to rise (triangles). (A) In the fmgE promoter region, three sites for cooperative binding of MrpC2 (light gray ovals) and FruA (dark gray ovals) are occupied in the order of their affinities for the two transcription factors, first site 3, then site 1, and finally site 2, resulting in activation of transcription. (B) In the fmgD promoter region, cooperative binding of two MrpC2 initially represses transcription, but eventually FruA outcompetes the downstream MrpC2 for binding to the upstream MrpC2, activating transcription. Activation of fmgD might occur at a slightly lower concentration of active FruA than activation of fmgE (see Discussion).