Altered prestarvation response in a nystatin resistant Dictyostelium discoideum mutant

Wild-type Dictyostelium amoebae secrete an autocrine, prestarvation factor (PSF) that allows them to measure the amount of food bacteria compared to t heir cell density. When the ratio of PSF to bacteria reaches a threshold, t he cells are signaled to prepare for eventual starvation. This prestarvatio n response (PSR) usually starts three to four generations before the end of exponential growth, leading to the accumulation of several aggregation spe cific genes during growth. We characterize a nystatin-resistant mutant, HK1 9, that expresses the PSR genes three generations earlier than wild type bu t has an otherwise wild-type PSR. Although HK19 has a full PSR during growt h, HK19 continues to grow at the wild-type rate and reaches normal cell den sities. Because HK19 temporally separates the PSR from starvation, it becam e possible to test whether starvation is required for development. Since HK 19 growing at low density can be induced to clump with either cAMP or folat e, it appears that the PSR and an external signal are sufficient for entry into development. These data suggest that the PSR is a complex genetic path way that induces genes involved in the exit from growth and the entry into development.