Figure 4 from Soderbom & Loomis, 1998 Proposed culmination network. Prestalk cells secrete the phosphopeptide SDF-1 (Ref. ). The response of prespore cells to SDF-1 is dependent on de novo protein synthesis, as shown by its sensitivity to the drug cycloheximide. A few hours later, prestalk cells secrete another peptide signal, SDF-2, in a manner dependent on the prestalk-specific membrane protein, TagC (Ref. ). Prespore cells respond to SDF-2 in a manner dependent on the histidine kinase DhkA. The signal transduction pathway initiated by DhkA leads to the inhibition of RegA. The response to SDF-2 can be blocked by the protein kinase A (PKA)-specific inhibitor, H89. This drug also inhibits release of SDF-2 in response to low levels of SDF-2 (Ref. ). This positive-feedback system is dependent on DhkA and results in the rapid release of SDF-2 from prestalk cells. When RegA is inhibited in prespore cells, cAMP accumulates and activates PKA, leading to rapid sporulation.