Abstract

The rnrB gene of Dictyostelium discoideum codes for the prespore specific small subunit of ribonucleotide reductase. The protein is expressed at two distinct times during the Dictyostelium discoideum developmental cycle; first at the onset of development (vegetative activity) and then again during late development at the slug stage (developmental activity). The $\beta$-galactosidase gene was used as a reporter gene and was fused to the noncoding area upstream of the rnrB gene. A series of $5\sp\prime$ and internal deletions were carried out in order to isolate the regulatory regions controlling gene expression. As well, primer extension was carried out on purified mRNA to determine the $5\sp\prime$ transcriptional start site of the rnrB gene. Results indicate that vegetative and developmental activities are regulated by separate promoter regions. Vegetative expression was mapped to an A/T rich region spanning $-$213 to $-$136. Developmental activity was found to be regulated by two G/C rich boxes, Box A ($-$238 to $-$221) and Box B ($-$313 to $-$299). Primer extension analysis of the transcript mRNA indicated a single transcriptional start site located 115 bases upstream of the start codon