Fig. 9. GskA and the regulation of stalk cell differentiation. The structure at the left is a representation of an early culminant, with prestalk cells undertaking the `reverse fountain' movement. As they pass the entrance to the stalk tube, the prestalk cells activate expression of the ecmB gene and are then termed pstAB cells (pink). Before entry to the stalk tube (blue), the ecmB promoter is inactive; high extracellular cAMP signalling acts (via the cAR3 cAMP receptor, the dual specificity kinase ZAK1 and GSK-3) to prevent the ecmB activator region (A) from functioning. In addition, Dd-STATa functions as a repressor, acting via two repressor elements (RI and RII) located distal to the ecmB cap site. At culmination, repression by Dd-STATa is also relieved by a parallel pathway involving activation of PKA.