PIGMENT ORIENTATION CHANGES ACCOMPANYING THE LIGHT STATE TRANSITION IN SYNECHOCOCCUS SP PCC-6301

Low temperature (77 K) linear dichroism spectroscopy was used to chara cterize pigment orientation changes accompanying the light state trans ition in the cyanobacterium, Synechococcus sp. PCC 6301 and those acco mpanying chromatic acclimation in Porphyridium cruentum in samples sta bilized by glutaraldehyde fixation. In light state 2 compared to light state 1 intact cells of Synechococcus showed an increased alignment o f allophycocyanin parallel to the cells' long axis whereas the phycobi lisome-thylakoid membrane fragments exhibited an increased allophycocy anin alignment parallel to the membrane plane. The phycobilisome-thyla koid membrane fragments showed less alignment of a short wavelength ch lorophyll a (Chi a) Qy transition dipole parallel to the membrane plan e in state 2 relative to state 1. To aid identification of the observe d Chl a orientation changes in Synechococcus, linear dichroism spectra were obtained from phycobilisome-thylakoid membrane fragments isolate d from red light-grown (increased number of PS II centres) and green l ight-grown (increased number of PS I centres) cells of the red alga Po rphyridium cruentum. An increased contribution of short wavelength Chi a Qy transition dipoles parallel to the long axis of the membrane pla ne was directly correlated with increased levels of PS II centres in r ed light-grown P cruentum. Our results indicate that the transition to state 2 in cyanobacteria is accompanied by an increase in the orienta tion of allophycocyanin and a decrease in the orientation of Chi a ass ociated with PS II with respect to the thylakoid membrane plane.