Dissecting interactions between nucleosides and germination receptors in Bacillus cereus 569 spores

Bacillus cereus 569 spores germinate either with inosine as a sole germinant or with a combination of nucleosides and L-alanine. Whereas the inosine-only germination pathway requires the presence of two different germination receptors (Gerl and GerQ) to be activated, the nucleoside/alanine germination pathway only needs one of the two receptors. To differentiate how nucleoside recognition varies between the inosine-only germination pathway and the nucleoside/ alanine germination pathway, we tested 61 purine analogues as agonists and antagonists of the two pathways in wild-type, [DELTA]gerl and [DELTA]gerQ spores. The structure-activity relationships of germination agonists and antagonists suggest that the inosine-only germination pathway is restricted to recognize a single germinant (inosine), but can be inhibited in predictable patterns by structurally distinct purine nucleosides. B. cereus spores encoding Gerl as the only nucleoside receptor ([DELTA]gerQ mutant) showed a germination inhibition profile similar to wild-type spores treated with inosine only. Thus, Gerl seems to have a well-organized binding site that recognizes inosine and inhibitors through specific substrate-protein interactions. Structure-activity analysis also showed that the nucleoside/alanine germination pathway is more promiscuous toward purine nucleoside agonists, and is only inhibited by hydrophobic analogues. B. cereus spores encoding GerQ as the only nucleoside receptor ([DELTA]gerl mutant) behaved like wild-type spores treated with inosine and L-alanine. Thus, the GerQ receptor seems to recognize substrates in a more flexible binding site through non-specific interactions. We propose that the Gerl receptor is responsible for germinant detection in the inosine-only germination pathway. On the other hand, supplementing inosine with L-alanine allows bypassing of the Gerl receptor to activate the more flexible GerQ receptor. DOI 10.1099/mic.0.030270-0