Mechanism of the light-state transition in photosynthesis

Linear-dichroism spectra of Anacystis nidulans at 77 K were determined for whole cells chemically fixed in light State 1 and light State 2. Whole cells were oriented by the squeezed gel technique using 5% gelatin 2.2 M sucrose gels. Peaks with positive dichroism were observed at 638 nm and 688 nm with shoulders at approx. 650 nm and 700 nm. The amplitude of the 650 nm shoulder was greater for cells in State 2 than those in State 1, and the State-2-minus-State 1 difference spectrum had a single peak at 656 nm. The linear dichroism spectrum of phycobilisomes isolated from A. nidulans showed peaks at 635 nm (phycocyanin) and 656 nm (allophycocyanin). The spectrum for thylakoid membranes free of phycobilisomes had one peak at 685 nm with a shoulder at 698 nm. We suggest that the change in dichroism at 656 nm between cells in State 1 and State 2 results from a change in orientation of the allophycocyanin core of the phycobilisome. This result is discussed in the context of our model for the light-state transition in phycobilisome-containing organisms.