FIG 12.

Current model of the pathway of RcGTA induction and the role of c-di-GMP. At a low starting concentration of cells, RcGTA structural genes are not expressed during the exponential phase of growth. As the concentration of cells increases and the culture approaches the stationary phase, a small percentage of cells escape from repression of the expression of the gafA gene (encoding an alternative sigma factor) by the extracellular protein Rc280. The GafA protein activates transcription of the RcGTA head and tail structural genes, as well as feeding back to amplify the transcription of the gafA gene itself. The CckA protein acts as a phosphatase because c-di-GMP levels are high, and the CtrA protein contributes to the induction of the gafA gene, leading to massive production of RcGTA head and tail proteins. Eventually, an unknown signal decreases the concentration of c-di-GMP, which changes CckA from a phosphatase to a kinase, and with an increasing concentration of CtrA∼P, the head spike and tail fiber genes are induced, as well as lysis genes leading to the release of mature RcGTA particles. In addition to the data in this paper, the model is supported by previous publications (21, 35, 39).