Throughout development, cells encounter extracellular signals that induce gene transcription, but is the cells' transcriptional response directly graded with signal concentration or a binary on-off switch? Now, Jonathan Chubb and colleagues report that the immediate-early transcriptional response in the social amoeba Dictyostelium discoideum is mainly binary (see ). During development, Dictyostelium cells choose between stalk and spore fates. Using an innovative fluorescent RNA construct that allows the live monitoring of nascent RNA, the researchers quantified the transcriptional response of Dictyostelium cells to different levels of the stalk fate-promoting extracellular signals DIF and cAMP. They found that the response is highly variable between cells, but that the strength of the individual transcriptional response, measured as fluorescence pulse duration, frequency and intensity, is largely unaffected by signal concentration. However, higher signal concentrations trigger transcription in more cells, indicating that individual cells have different signal concentration thresholds. From their results, the authors suggest that the Dictyostelium transcriptional response is mainly binary and speculate that this is conserved in other developmental contexts.
