Characterization of two highly diverged but developmentally co-regulated cysteine proteinase genes in Dictyostelium discoideum.

The cysteine proteinase 1 and 2 mRNA sequences of Dictyostelium discoideum encode proteins with a high degree of homology to plant and animal sulphydryl proteinases. The two mRNA sequences are co-ordinate in their regulation, both being first expressed late during cellular aggregation, prematurely induced in response to exogenous cAMP and several-fold enriched in prestalk over prespore cells. The two proteins are considerably diverged, with only 43% overall homology but all residues known to be important in catalysis are conserved and both contain a hydrophobic leader peptide which forms part of an N-terminal domain of just over 100 amino acids not found in the mature form of known cysteine proteinases. We have determined the sequence organization of both genes and find differences both in the number and position of introns. The close co-regulation of these two genes suggests that they may play a common role in Dictyostelium development, presumably in the autodigestion of cellular protein which occurs during differentiation. However, the low degree of sequence homology and major differences in gene organization indicate that they have undergone a considerable period of separate evolution and that they may differ in their precise function.