Abstract

Growth and development are mutually exclusive phases of the life cycle of the cellular slime mould Dictyostelium discoideum . The amoebae grow as single cells and divide by binary fission until deprived of nutrients. Starvation forces the cells to initiate development where they organize into multicellular structures, each consisting of a spore head supported by a stalk. Prior to terminal differentiation, the prespore cells undergo a round of DNA synthesis, but it is not known whether this DNA synthesis takes place in the nucleus or the mitochondria. Ribonucleotide reductase (RNR) converts ribonucleotides to deoxyribonucleotides and is the rate-limiting step of DNA synthesis. The expression of this enzyme has been temporally and spatially correlated with DNA synthesis and the expression may therefore be used as a marker for DNA synthesis. Dictyostelium is unusual among eukaryotes in that it harbours genes encoding two classes of RNR enzymes. Using a short-lived Ý-galactosidase as a reporter, we have found the two subunits of the Class I RNR to be mainly expressed in prespore cells. The subcellular localisation of the RNRs was examined during growth and early development using fluorescent fusion proteins. While the subunits of the Class I RNR are spatially separated, the Class II enzyme resides in the cytoplasm. Using gene replacement strategy to examine functions, we have shown that the gene encoding the Class II RNR is not essential for Dictyostelium growth and development.