We identified signals that affect mRNA levels complementary to a gene that is highly expressed in vegetative Dictyostelium discoideum cells. This gene has been cloned as cDNA in the plasmid pcD-D2. The level of transcripts homologous to pcD-D2 fell dramatically in strain XP55 during the aggregation stage of development when cells differentiate on agar. The level, however, did not fall simply as a result of starvation or aggregation-specific cell contact. Rather, before the level is reduced cells must be deprived of amino acids and cyclic AMP administered in amounts and at intervals in pulses to mimic cyclic AMP signal-relay in aggregation. This effect can be blocked either with cyclic AMP-S (a non-hydrolysable cyclic AMP analogue) or adenosine, both of which prevent cyclic AMP binding to the cyclic AMP cell surface receptor. It is also blocked in ‘frigid’ aggregation-deficient mutants HC85 and HC112 known to be defective in a G alpha protein. We conclude that the transcript level is balanced by positive nutritional signals acting against negative signals transduced in part through a cell surface cyclic AMP receptor.

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