Howard Hughes Medical Institute, Department of Biochemistry and Cell Biology MS-140, Rice University, Houston, Texas 77005-1892, USA.
Developing Dictyostelium cells form large aggregation streams that break up into groups of 0.2 x 10(5) to 1 x 10(5) cells. Each group then becomes a fruiting body. smlA cells oversecrete an unknown factor that causes aggregation streams to break up into groups of approximately 5 x 10(3) cells and thus form very small fruiting bodies. We have purified the counting factor and find that it behaves as a complex of polypeptides with an effective molecular mass of 450 kD. One of the polypeptides is a 40-kD hydrophilic protein we have named counting. In transformants with a disrupted counting gene, there is no detectable secretion of counting factor, and the aggregation streams do not break up, resulting in huge (up to 2 x 10(5) cell) fruiting bodies.