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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 |
