No abstract

The cellular slime mold Dictyostelium discoideum, a lower eukaryote feeding on bacteria in soil, is highly resistant to DNA damaging agents. D. discoideum expresses at least two RecA homologs, Rad51 and mitochondria localizing RecA. Rad51 consisting of 351 amino acid residues is 69% identical to human Rad51. UV light-irradiation and H2O2-treatment increased the expression level of Rad51, suggesting that Rad51 is required for DNA repair in D. discoideum. A gene disruption mutant was not viable, suggesting that Rad51 is essential for D. discoideum.

In the cellular slime mold Dictyostelium discoideum, Countin2 and Countin3 proteins are thought to limit the minimum size of the multicellular structure since countin2 -and countin3 -strains form small fruiting bodies. Expression levels of the cell-cell adhesion proteins, gp24 and gp80, in wild type and null mutants were compared. During the process of aggregation, countin2 -cells expressed lower levels of gp24 and gp80 than those expressed by the wild type. The expression levels of gp24 and gp80 in countin3 -cells were almost the same as those in wild-type cells in the early stages of aggregation but were lower than wild-type levels in the late stages. These results indicate that Countin2 and Countin3 proteins enhance the expression of gp24 and gp80 in D. discoideum in different ways.

The cellular slime mold Dictyostelium discoideum expresses a RecA homolog that is distributed in mitochondria. A gene disruption mutant is hypersensitive to DNA-damaging agents, indicating that RecA is required for the repair of mitochondrial DNA. We analyzed the process by which a multicellular structure of recA -cells is formed. Four hours after starvation, the internal glucose level of recA -cells (11.8±0.9 nmol/mg protein) was higher than that of wild-type cells (6.7±0.1 nmol/mg protein). The degree of cell-cell adhesion of recA -cells was greater than that of wild-type cells, while the migration speed of recA -cells (3.5±0.2 mm/min) was less than that of wild-type cells (4.3±0.3 mm/min). The increase in cell-cell adhesion and decrease in motility of recA -cells gave rise to the formation of large multicellular structures. The diameters of mounds formed by recA -and wildtype cells were 0.30±0.01 mm and 0.17±0.01 mm, respectively.