Competition-integration of blue and orange stimuli in Halobacterium salinarum cannot occur solely in SRI photoreceptor

Experiments on the integration of blue and orange stimuli in Halobacterium salinarum were performed by using different combinations of blue and orange steps. The results show that the prevalence of the blue stimulus over the orange one depends on both the blue and the orange light intensities. A qua ntitative analysis of the current hypotheses on the phototransduction of or ange and UV-blue light stimuli is presented, showing that the balancing bet ween the two antagonistic stimuli should depend only on the intensity of th e blue stimulus and not on that of the orange one, provided that the combin ation of the two stimuli occurs linearly at the photoreceptor stage. We con clude that blue and orange stimuli elicit distinct intracellular signals wh ose integration occurs downstream of the photoreceptor.