A novel gene, DD3-3, from Dictyostelium discoideum has been isolated by an mRNA differential display between a wild-type strain AX2 and a mutant HG794 which is defective in O-glycosylation. Functional analysis of the novel gene, DD3-3, was conducted by preparing a knockout mutant, DD3-3KO, and a GST:DD3-3 fusion protein. The mutant DD3-3KO cells were allowed to develop about 1.5 h earlier than the wild-type strain AX2 cells. Northern blotting analysis of the knockout mutant cells showed a remarkable downregulation of Reg A, cAMP-dependent phosphodiesterase, and overexpression of protein tyrosine kinase (PTK) during early development and its shutdown during late development. The relationship between O-glycosylation and phosphorylation involving Reg A gene is discussed.