The Bacillus subtilis germinant receptor GerA triggers premature germination in response to morphological defects during sporulation

Abstract

During sporulation in Bacillus subtilis, germinant receptors assemble in the inner membrane of the developing spore. In response to specific nutrients these receptors trigger germination and outgrowth. In a transposon-sequencing screen, we serendipitously discovered that loss of function mutations in the gerA receptor partially suppress the phenotypes of >25 sporulation mutants. Most of these mutants have modest defects in the assembly of the spore protective layers that are exacerbated in the presence of a functional GerA receptor. Several lines of evidence indicate that these mutants inappropriately trigger activation of GerA during sporulation resulting in premature germination. These findings led us to discover that up to 8% of wild-type sporulating cells trigger premature germination during differentiation in a GerA-dependent manner. This phenomenon was observed in domesticated and undomesticated wild-type strains sporulating in liquid and on solid media. Our data indicate that the GerA receptor is poised on a knife's edge during spore development. We propose that this sensitized state ensures a rapid response to nutrient availability, but also elicits premature germination of spores with improperly assembled protective layers resulting in the elimination of even mildly defective individuals from the population.

Electrical-charge accumulation enables integrative quality control duringB. subtilissporulation

“…As expected, the deletion of gerA gene eliminates the change of phase brightness with forespores (Figure 3A; lower panel, Figure 3B, and Movie 5). This observation appeared complementary to the genetic analyses of a recent paper (18). This study by Ramirez-Guadiana et al elegantly demonstrated that GerA-mediated premature germination can act as a quality-control system of forespore morphogenesis; i.e .…”

“…While a previous study suggested that the frequency of premature germination may be constant in different media (18), our model predicted that environmental conditions, that affect the membrane potential, should alter the frequency of premature germination. To this end, we focused on glutamate availability because glutamate is a gating molecule for the B. subtilis potassium channels which remain closed in glutamate rich media (2, 4).…”

“…Ramirez-Guadiana et al reported that the most dramatic increase in premature germination was observed with the deletion of ylbJ (later renamed as spoVV ) (18). SpoVV, a concentrative nucleoside transporter, translocates dipicolinic acid (DPA; pyridine-2,6- dicarboxylic acid) against the concentration gradient across the outer forespore membrane from the mother-cell compartment to the intermembrane space (32).…”

“…Based on the insight we gained in this study, we suspect that the spatio-temporal organization of sporulation during biofilm formation may also be regulated by electrical signaling and premature germination. Given that premature germination has been seen as a type of programmed cell death (18, 37), it may also provide some benefits to the surrounding cells by providing nutrients or physical space. Such interactions would be pronounced when cells are structurally organized, such as in biofilms (38, 39).…”

“…While it is well established that sporulation is regulated by a complex genetic program, it is also evident that the process is subject to a variety of internal and external conditions (Figure 1A). Poorly developing spores are eliminated from the population through quality control systems mediated by Clp proteases and GerA-dependent premature germination (17, 18). Intriguingly, the quality ( e.g.…”