SPIF regulates spore coat protein synthesis

Spore coat protein synthesis in submerged pseudoplasmodia of Dictyostelium discoideum is dependent on the presence of a low relative molecular mass extracellular factor, SPIF, the activity of which can be mimicked by methionine. In vitro translation and northern blot analysis revealed that the level of spore coat protein mRNA in pseudoplasmodia incubated in the absence of methionine is little different from that in its presence. Furthermore, nogalamycin, a potent inhibitor of RNA synthesis, does not prevent the regulation of spore coat protein synthesis by methionine. These data suggest that the regulatory step is probably at the translational level. The proportion of total ribosomes associated in polysomes in pseudoplasmodia incubated in the absence of methionine is substantially lower than in its presence indicating a relative decrease in the number of translationally active mRNAs. However, measurements of the average polysome size and ribosome transit time in pseudoplasmodia initiated in the presence or absence of methionine show that the initiation rate of protein synthesis is essentially identical in both situations.