Ribosomal protein gene expression is cell type specific during development in Dictyostelium discoideum

Starvation for amino acids initiates the developmental cycle in the cellular slime mold, Dictyostelium discoideum. Upon starvation one of the earliest developmental events is the selective loss of the ribosomal protein mRNAs from polysomes. This loss depends upon sequences in the 5' non-translated leader of the ribosomal protein (r-protein) mRNAs. Here evidence is presented which indicates that those cells which will become prestalk cells express the ribosomal protein genes during development under starvation conditions. Cells which enter the prespore pathway shut off r-protein synthesis. The promoter and 5' non-translated leader sequences from two ribosomal protein genes, the rp-L11 and the rp-S9 genes, are fused to the Escherichia coli beta-galactosidase reporter gene. While beta-galactosidase enzyme activity is detected in situ in most growing cells, by 15 h of development beta-galactosidase enzyme activity is largely lost from the prespore cells although strong beta-galactosidase enzyme activity is present in the prestalk cells. These observations suggest the possibility that the ribosomal protein mRNAs are excluded from polysomes in a cell-type-specific manner.