Dynamic aspects of phycobilisome structure

Abstract

In exponentially growing cells of Synechococcus sp. 6301, over 95% of the phycobiliproteins are located in phycobilisomes, and the remainder is present in the form of low molecular weight aggregates. In addition to the subunits of the phycobiliproteins (C-phycocyanin, allophycocyanin, allophycocyanin B), the phycobilisomes of this unicellular cyanobacterium contain five non-pigmented polypeptides. During the initial phase of starvation (24 h after removal of combined nitrogen from the growth medium), the phycobiliproteins in the low molecular weight fraction largely disappeared. Phycocyanin was lost more rapidly from this fraction than allophycocyanin. Simultaneous changes in the phycobilisome were (1) a decrease in sedimentation coefficient, (2) a decrease in phycocyanin: allophycocyanin ratio, (3) a shift in the fluorescence emission maximum from 673 to 676 nm, and (4) a selective complete loss of a 30,000 dalton non-pigmented polypeptide. Upon extensive nitrogen starvation (72 h), the intracellular level of phycocyanin decreased by over 30-fold. These results indicate that in the early stage of nitrogen starvation, the free phycobiliproteins of the cell are degraded, as well as a significant proportion of the phycocyanin from the periphery of the phycobilisome. However, the structures partially depleted of phycocyanin still function efficiently in energy transfer. On extended starvation, total degradation of residual phycobilisomes takes place, possibly in conjunction with the detachment of these structures from the thylakoids. None of the effects of the absence of combined nitrogen were seen when cells were starved in the presence of chloramphenicol, or in a methionine auxotroph starved for methionine.