Over the last four decades Ger-type receptors have been studied in Bacillus subtilis 1, Bacillus megaterium 3, 4, Bacillus cereus 5, 6, Bacillus anthracis 7, 8, Bacillus licheniformis 9, 10, Clostridium botulinum, Clostridium sporogenes 11, and Clostridium perfringens 12, 13. Germination does not require the active transport of these molecules into the spore and it is therefore thought that the Ger receptors function as nutrient sensors 2. These receptors have been implicated in the response to a wide array of nutrients including amino acids, sugars, nucleosides, and inorganic cations. Most spore-forming bacteria from the orders Bacillales and Clostridiales contain several operons that encode putative 3-subunit receptors of the Ger family 1. These data suggest that GerAB is the L-alanine sensor and that B subunits in this broadly conserved family function in nutrient detection. Finally, substitutions of bulkier residues at these positions cause constitutive germination. We show that mutations in gerAB predicted to disrupt the ligand-binding pocket impair germination, while mutations predicted to function in L-alanine recognition enable spores to respond to L-leucine or L-serine. Using evolutionary co-variation analysis, we provide evidence that GerAB adopts a structure similar to an L-alanine transporter from this superfamily. GerAB belongs to the Amino Acid-Polyamine-Organocation superfamily of transporters. The prototypical receptor from Bacillus subtilis consists of three proteins (GerAA, GerAB, GerAC) required for germination in response to L-alanine. How spores monitor nutrients is poorly understood but in most cases requires putative membrane receptors.
Bacteria from the orders Bacillales and Clostridiales differentiate into stress-resistant spores that can remain dormant for years, yet rapidly germinate upon nutrient sensing.
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