Spore coat protein synthesis during development of Dictyostelium discoideum requires a low-molecular-weight inducer and continued multicellularity.

The major spore coat proteins of Dictyostelium discoideum are synthesized during the culmination stage of development. In an attempt to examine the regulatory mechanisms involved, spore coat protein synthesis by pseudoplasmodia harvested prior to culmination and incubated in submerged culture under various environmental conditions has been monitored. It is reported that the synthesis of spore coat proteins SP170, SP103, SP94, SP82, SP76, and SP55 is dependent upon the presence of a low-molecular-weight (Mr approx 100), heat-stable factor secreted by cells incubated at high density in buffer. Previous studies have implicated cyclic AMP, ammonia, and amino acids in spore cell differentiation. Partial purification of the spore coat protein inducing factor (SPIF), together with attempts to mimic its activity, indicate that SPIF is not identical with any of these molecules and it is probably also distinct from DIF and "fruit juice," two other factors which regulate the spore-stalk decision and the initiation of culmination, respectively, in D. discoideum. In addition to SPIF, the continued expression of the spore coat protein genes also requires that the integrity of the pseudoplasmodium be maintained. Unlike the expression of many other genes after aggregation, this latter requirement cannot be replaced by exogenous cyclic AMP. Termination of spore coat protein gene expression occurs despite the presence of excess exogenous SPIF and hence involves mechanisms other than the destruction or depletion of SPIF.