Modeling the functioning of YtvA in the general stress response in Bacillus subtilis.

The blue-light photoreceptor YtvA activates the general stress response (GSR) of Bacillus subtilis by activating a large protein complex (the stressosome). We have constructed a model for the YtvA's photocycle, and derived an equation for the fraction of YtvA in the light-induced signaling state at a given light intensity. The model was verified experimentally in vitro on wild type YtvA and on an R63K mutant with faster recovery kinetics. Application of the model to the activation of the GSR at various light intensities in vivo revealed that the GSR is more sensitive to light than would be expected based on YtvA's in vitro kinetics. These results were confirmed with the R63K mutant and a slower-recovering V28I mutant. Additionally, we have demonstrated the presence of a near-UV-light-induced branching reaction that converts the signaling state of YtvA to the dark state. Extension of the model with this reaction shows that it does not contribute significantly to the in vivo blue-light response. The model represents an important step towards a complete systems biology model of the GSR.