Role of Ger Proteins in Nutrient and Nonnutrient Triggering of Spore Germination in Bacillus subtilis

Dormant Bacillus subtilis spores germinate in the presence of particular nutrients called germinants. The spores are thought to recognize germinants through receptor proteins encoded by the gerA family of operons, which includes gerA, gerB, and gerK. We sought to substantiate this putative function of the GerA family proteins by characterizing spore germination in a mutant strain that contained deletions at all known gerA-like loci. As expected, the mutant spores germinated very poorly in a variety of rich media. In contrast, they germinated like wild-type spores in a chemical germinant, a 1-1 chelate of Ca(2+) and dipicolinic acid (DPA). These observations showed that proteins encoded by gerA family members are required for nutrient-induced germination but not for chemical-triggered germination, supporting the hypothesis that the GerA family encodes receptors for nutrient germinants. Further characterization of Ca(2+)–DPA-induced germination showed that the effect of Ca(2+)–DPA on spore germination was saturated at 60 mM and had a K(m) of 30 mM. We also found that decoating spores abolished their ability to germinate in Ca(2+)–DPA but not in nutrient germinants, indicating that Ca(2+)–DPA and nutrient germinants probably act through parallel arms of the germination pathway.