Cell type specificity and mechanism of control of a gene may be reverted in different strains of Dictyostelium discoideum

Twelve genes which are expressed exclusively in pre-spore cells of Dictyost elium strain AX3 are expressed exclusively in pre-stalk cells of strain AX2 . One gene has the opposite behavior: it is expressed in pre-stalk cells in AX3 and in pre-spore cells in AX2. The change in cell type specificity inv olves a change in the mechanism of control of gene expression. When they ar e expressed in pre-stalk cells, genes are controlled at the level of transc ription, whilst in pre-spore cells, they are controlled at the level of mRN A stability. Genes expressed in prestalk cells in strain AX2, fused with an AX2 pre-spore specific promoter, become regulated at the level of mRNA sta bility. These findings indicate that at least a group of pre-stalk mRNAs po ssess the cis-destabilizing element typical of pre-spore mRNAs, though they are not destabilized in disaggregated cells. This is due to the fact that ribosomal protein S6, phosphorylation of which is responsible for controlli ng the stability of pre-spore mRNAs, is not dephosphorylated in disaggregat ed pre-stalk cells. These cells lack an S6 phosphatase activity which has b een purified from disaggregated pre-spore cells. (C) 2000 Elsevier Science B.V. All rights reserved.