Bacterial spores can rapidly exit dormancy through the process of germination. This process begins with the activation of nutrient receptors embedded in the spore membrane. The prototypical germinant receptor in
Bacillus subtilis
responds to L-alanine and is thought to be a complex of proteins encoded by the genes in the
gerA
operon:
gerAA
,
gerAB
, and
gerAC
. The GerAB subunit has recently been shown to function as the nutrient sensor, but beyond contributing to complex stability, no additional functions have been attributed to the other two subunits. Here, we investigate the role of GerAA. We resurrect a previously characterized allele of
gerA
(termed
gerA*
) that carries a mutation in
gerAA
and show it constitutively activates germination even in the presence of a wild-type copy of
gerA
. Using an enrichment strategy to screen for suppressors of
gerA*
, we identified mutations in all three
gerA
genes that restore a functional receptor. Characterization of two distinct
gerAB
suppressors revealed that one (
gerAB[E105K])
reduces the GerA complex's ability to respond to L-alanine, while another (
gerAB[F259S]
) disrupts the germinant signal downstream of L-alanine recognition. These data argue against models in which GerAA is directly or indirectly involved in germinant sensing. Rather, our data suggest that GerAA is responsible for transducing the nutrient signal sensed by GerAB. While the steps downstream of
gerAA
have yet to be uncovered, these results validate the use of a dominant-negative genetic approach in elucidating the
gerA
signal transduction pathway.
Importance
Endospore formers are a broad group of bacteria that can enter dormancy upon starvation and exit dormancy upon sensing the return of nutrients. How dormant spores sense and respond to these nutrients is poorly understood. Here, we identify a key step in the signal transduction pathway that is activated after spores detect the amino acid L-alanine. We present a model that provides a more complete picture of this process that is critical for allowing dormant spores to germinate and resume growth.
Genetic evidence for signal transduction within the
Bacillus subtilis
GerA germinant receptor
