Division of Bioengineering, Faculty of Engineering, Toyama University, Toyama 930-8555, Japan.
Countin, a cell-counting factor in Dictyostelium discoideum, is considered to limit the maximum size of the multicellular structure, because a countin null strain forms a huge fruiting body compared to that of the wild-type. A novel gene, countin2, that is highly homologous to countin (40% identity in amino acid sequence) was identified in the D. discoideum genome. The countin2 null strain formed a 1.7-fold higher number of the aggregates, resulting in smaller fruiting bodies compared with those of wild-type cells. Thus, the Countin2 protein is thought to limit the minimum size of the multicellular structure. The size and number of aggregates formed by a mixture of countin null and countin2 null strains were the same as those of the wild-type. These findings demonstrate that a combination of Countin and Countin2 proteins determines the appropriate size of the multicellular structure of D. discoideum.