A characterization of the preaggregative period of Dictyostelium discoideum

The preaggregative period of Dictyostelium discoideum has been characterized by measuring the reduction in time for the onset of aggregation under conditions which hinder close cell-cell associations, inhibit protein synthesis, and/or include continuous high concentrations or pulsed low concentrations of exogenous cAMP. The results demonstrate that: the preaggregative period (normally 7 hr for cells from log phase cultures) can be dissected into two distinct components: an initial component which includes the first 4.5 hr, and a second component which includes the last 2.5 hr; the first component will progress at normal rate in the continuous absence of close cell-cell associations (as single amoebae in suspension) or in the continuous absence of de novo protein synthesis; the second component will not progress in the continuous absence of close cell-cell associations or de novo protein synthesis; high concentrations of cAMP continuously present in suspension cultures do not affect progress through the first component, nor do they support progress through the second component; however, if cells are allowed to form close cell-cell associations during progress through the first component, high concentrations of cAMP will support progress through the second component in the absence of close cell-cell associations; these associations, which render cells sensitive to cAMP, will occur in the absence of de novo protein synthesis and before the acquisition of contact sites A; these associations may be completely bypassed if suspended cells are continuously pulsed with low concentrations of cAMP; in this case, pulses of cAMP will support progress through the final component in continuous suspension cultures; and the acquisition of contact sites A will not occur in the absence of progress through the second component; in contrast, the acquisition of cAMP binding sites on the cell's surface will occur. These results are considered in terms of the complexity and regulation of the preaggregative period of Dictyostelium.