Pigment orientation and excitation energy transfer in Porphyridium cruentum and Synechococcus sp. PCC 6301 cross-linked in light state 1 and light state 2 with glutaraldehyde

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Phycobilisome/thylakoid membrane fragments were isolated from the red alga Porphyridium cruentum and the cyanobacterium Synechococcus sp. PCC 6301. Intact cells were cross-linked with glutaraldehyde in light state 1 or light state 2, broken with a French press and membrane fractions isolated by sucrose density gradient centrifugation. For both species a fraction was isolated which contained phycobilisomes energetically coupled to the thylakoid membrane. The 77 K fluorescence emission spectra of these phycobilisome/thylakoid membrane fragments retained the relative PS II and PS I Chl a fluorescence emission yields characteristic of the light state in which the cells had originally been cross-linked. A comparison of linear dichroism spectra of the isolated phycobilisome / membrane fragments with intact cells showed that the significant change in allophycocyanin orientation previously reported to accompany the state transition in the cyanobacteria Synechococcus sp. PCC 6301 (Bruce, D. and Biggins, J. (1985) Biochim. Biophys. A eta 810, 295-301) was not observed in the isolated membrane fragments. Linear-dichroism spectra of the membrane fragments from P. cruentum in state 1 and state 2 showed a smallchange in phycoerythrin orientation but no change in allophycocyanin orientation. As observation of the change in allophycocyanin orientation was dependent on the integrity of the intact cell, we conclude that the change reflects an orientation with respect to the long axis of the cell and not an orientation with respect to the thylakoid-membrane plane.