Fig. 8.
Model for the control of spo0A expression. The upper panel shows that during the exponential phase of growth, O2 represses transcription from Ps. Thus, transcripts originating from Pv are the chief source of mRNA, which due to secondary structure is impaired in translation. The dotted circle conveys that the identity of the molecule bound at O2 has not been confirmed as being Spo0A∼P biochemically. The lower panel shows that during the transition to stationary phase in sporulation-inducing medium, Pv is repressed by Spo0A∼P bound at O1, and Spo0A∼P molecules bound at O3 and downstream activate Ps, overriding repression by O2. Cooperative binding of Spo0A∼P molecules renders the switch from Pv to Ps highly sensitive to small changes in Spo0A∼P levels. The rate of Spo0A synthesis is enhanced both by the switch to the strong Ps promoter and the enhanced translatability of mRNA originating from Ps.