Abstract
Phosphorylated Spo0A is a master regulator of stationary phase development in the model bacterium Bacillus subtilis, controlling the formation of spores, biofilms, and cells competent for transformation. We have monitored the rate of transcription of the spo0A gene during growth in sporulation medium using promoter fusions to firefly luciferase. This rate increases sharply during transient diauxie-like pauses in growth rate and then declines as growth resumes. In contrast, the rate of transcription of an rRNA gene decreases and increases in parallel with the growth rate, as expected for stable RNA synthesis. The growth pause-dependent bursts of spo0A transcription, which reflect the activity of the spo0A vegetative promoter, are largely independent of all known regulators of spo0A transcription. Evidence is offered in support of a “passive regulation” model in which RNA polymerase stops transcribing rRNA genes during growth pauses, thus becoming available for the transcription of spo0A. We show that the bursts are followed by the production of phosphorylated Spo0A, and we propose that they represent initial responses to stress that bring the average cell closer to the thresholds for transition to bimodally expressed developmental responses. Measurement of the numbers of cells expressing a competence marker before and after the bursts supports this hypothesis. In the absence of ppGpp, the increase in spo0A transcription that accompanies the entrance to stationary phase is delayed and sporulation is markedly diminished. In spite of this, our data contradicts the hypothesis that sporulation is initiated when a ppGpp-induced depression of the GTP pool relieves repression by CodY. We suggest that, while the programmed induction of sporulation that occurs in stationary phase is apparently provoked by increased flux through the phosphorelay, bet-hedging stochastic transitions to at least competence are induced by bursts in transcription.
Highlights
In response to nutritional deprivation and high population density, the model bacterium Bacillus subtilis can embark on several developmental programs leading to sporulation, cannibalism, biofilm formation and genetic competence [1,2,3,4]
A hallmark of the intensively studied model organism Bacillus subtilis is its ability to enter developmental pathways: forming spores, acquiring the ability to take up environmental DNA, and the formation of biofilms
We have monitored the rate of transcription of spo0A during growth and have detected unexpected fluctuations that correlate with pauses in the growth rate
Summary
In response to nutritional deprivation and high population density, the model bacterium Bacillus subtilis can embark on several developmental programs leading to sporulation, cannibalism, biofilm formation and genetic competence [1,2,3,4]. These exquisitely regulated responses anticipate starvation, initiating before crucial metabolic pools are exhausted. High affinity promoters respond early and with kinetic heterogeneity, i.e. some cells respond earlier than others [6,7] This probably reflects kinetic heterogeneity in the accumulation of Spo0A,P, which plays an important biological role [7,8,9]. In important reports it has recently been shown that the rate limiting process in the early stages of sporulation is the flux of phosphate through the phosphorelay, rather than the synthesis of Spo0A protein [7,10]
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