Identification of cis-regulating elements and trans-acting factors regulating the expression of the gene encoding the small subunit of ribonucleotidereductase in Dictyostelium discoideum

We have examined the promoter of rnrB, the gene encoding the small subunit of ribonucleotide reductase of Dictyostelium discoideum, using lacZ as a re porter gene. Deletion analysis showed that expression of this gene in veget ative cells involves an AT-rich element, whereas its expression in prespore cells during development requires a region encompassing two G/C-rich eleme nts, designated box A and box B, Removal of boxes A and B results in very l ow level of activity. When either box A or box B is deleted, prestalk. cell s adjacent to the prespore zone also express beta-galactosidase, The behavi or of these cis-regulatory elements implies that the mechanism regulating t he prespore-specific expression of rnrB is different from that regulating o ther known prespore genes. We have used electrophoretic mobility shift assays to identify factors that interact with box A and box B, Box A interacts with a factor that is found in the nuclear fraction. While box B interacts with a factor that is prese nt in the cytosolic fraction throughout growth and development, its presenc e in the nuclear fraction is developmentally regulated, Results from compet ition assays suggest that both box A and box B interact with transcriptiona l activators that have not been characterized previously.