Regulates both cAMP signaling during early development and the stress response. Functions as an activator of adenylylcyclase. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Expressed specifically at aggregation-stage. Undergoes autophosphorylation in the catalytic domain. Cells exhibit reduced viability under hyperosmotic conditions. They produce smaller aggregates on membrane filters and do not form aggregation streams on a plastic surface under submerged starvation conditions, but are normal in sexual development. During early asexual development, the expression of cAMP related genes peaks earlier in the knockout mutants. Neither cAMP oscillation in starved cells nor an increase in the cAMP level following osmotic stress was observed in mutants lacking krsA. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.