UC San Diego

Cellular differentiation is a crucial feature of developmental processes in diverse organisms. Endospore formation represents a developmental pathway entailing the transformation of rapidly dividing vegetative cells into metabolically dormant and highly resilient spores. Sporulation commences with polar cell division, which divides the sporangium into the mother cell (larger) and forespore (smaller). Despite initially having similar molecular compositions, the two cells activate cell-autonomous programs of gene expression, causing their fates to diverge. The mother cell engulfs the forespore and lyses to liberate the mature spore. Upon sensing nutrient availability, spores readily escape dormancy, resuming growth within hours.

One of the few remaining deficiencies in the knowledge of sporulation is the developmental role played by proteins synthesized during vegetative growth. Although these proteins comprise the majority of the proteome, the lack of approaches to inactivate proteins in a spatiotemporally controlled manner without perturbing vegetative growth or sporulation initiation has prevented a systematic interrogation of their contribution to spore formation and revival.

We have developed a versatile genetic framework that harnesses cell-specific gene expression to target proteins for degradation in a spatiotemporally regulated manner, thereby allowing us to investigate the role of metabolic enzymes during development. We have employed this methodology to document the metabolic interactions underlying the spore’s transition to dormancy, finding that the forespore becomes dependent on mother cell-derived metabolic precursors for biosynthesis. Our results empirically validate a long-standing model that the mother cell assumes the biosynthetic burden and nurtures the forespore with metabolites through a feeding tube-like apparatus connecting the two cells.

Our results also suggest that the metabolic capabilities of the forespore are drastically reprogrammed in preparation for dormancy, yielding a profound metabolic differentiation of the mother cell and forespore. Enzymes functioning in diverse biosynthetic pathways are rapidly depleted from the forespore. We have found that YjbA governs this pathway by targeting key metabolic enzymes for ClpCP-mediated proteolysis in the forespore, and that it is required for the efficient emergence from cellular dormancy under stress conditions. Altogether, our methodology has enabled the dissection of many facets of spore formation and revival, and will be instrumental in future work.