IMPROVED 5-STEP MODELING OF THE PHOTOSYSTEM-II S-STATE MECHANISM IN CYANOBACTERIA

We present a model of the S-state mechanism, as well as an improved ei genvalue analysis, that integrate into a coherent ensemble several fea tures found since the S-state model was initially developed. These fea tures include the presence of S-1, deactivations in the dark interval between flashes, and the change in the number of active PS II centers by photoinhibition or photoactivation. A new feature is the capacity t o predict the steady-state distribution of S-states under conditions o f steady photoinhibition or photoactivation. The improved eigenvalue a nalysis allowed the calculation of the initial S-state distribution. I n addition, the model resolved 'true' photochemical misses from appare nt misses due to deactivations in the dark interval between flashes. T he model suggested that most of the misses that are commonly reported are due to deactivations, and not to an intrinsic inefficiency of the photochemical mechanism of PS II. Because models that allow double-hit s encompassing the S-2 to S-3 transition Often predict negative initia l quantities of S-2 in cyanobacteria, our proposed model specifically prohibited them. The model accounts for inhomogeneous misses and a ste ady-state distribution of the type (S-2)approximate to(S-1)>(S-3)appro ximate to(S-0). This 5-step model uses only 4 probabilities, and is th erefore easy to handle. The use of this model is critical for the anal ysis of several cyanobacterial strains, as well as for any species tha t show non-negligible deactivations in the dark interval between flash es.