Phycobilisome Disassembly

Cyanobacteria can be exposed to changes in their typical growth conditions that modify both the number and characteristics of the phycobilisome. As already noted, the expression of the genes encoding for phycobilisome components can be altered by external environmental changes within a short period of time. However, more long^erm changes that require changes in the phycobilisome makeup may occur. One such change is the response of cyanobacteria to the lack of specific nutrients, especially nitrogen and sulfur. Under these conditions, the cyanobacterial cells change color from blue- green to yellow-green in a process known as chlorosis, or bleaching [81]. Using spectroscopy, and other techniques, it was found that the bleaching occurs within a few hours of the onset of nutrient starvation and was a result of the loss of the phycobilisome in an ordered fashion [55]. It has been proposed that the disassembly of the phycobilisome can have two beneficial roles. During starvation the cellular metabolism decreases, while the rate of photosynthesis may be unchanged. This imbalance could lead to excessive absorption of excitation energy, which then leads to the production of harmful radical species [82]. Under extreme conditions this may lead to cell death and must be avoided. Since the rate of photosynthesis is directly coupled to the rate of energy absorption, uncoupling and degradation of the major antenna will be highly beneficial in avoiding overexcitation. A second role for phycobilisome degradation is to serve as an internal nutrient reservoir. The phycobilisome can supply the cell with amino acid residues for the synthesis of protein systems required for high affinity nutrient uptake. Additionally, by further degradation, it can supply building blocks for the synthesis of other metabolites or as a source of energy. Collier and Grossman used a genetic screen to identify mutants unable to degrade phy-cobilisomes during nutrient starvation in order to identify the molecular pathway of phycobilisome degradation ,83]. The non-bleaching A, nblA, gene was thus identified and has been found to be present in all cyanobacteria. Nitrogen is the major nutrient whose lack can lead to a 50 fold increase in the amount of NblA protein ,83], In some cyanobacteria, such as Synechococcus sp. PCC 7942, sulfur and phosphorus limitation can also induce the nblA response [83]; while, in other species, sulfur limitation does not influence the expression of nblA [84] . During starvation, wild-type cells, but not nblA mutants, degrade two rod-linker proteins, Lr 33 and LR 34.5, indicating a possible functional connection between the linkers and the NblA protein [84]. This observation supports previous data showing that wild-type cells degrade the phycobilisome by first decreasing the length of the rods [28, 85]. It was shown that in the cyanobacterium Tolypothrix PCC 7601, the NblA protein has affinity for both PC and PE subunits, but not for APC or for PBPs from other cyanobacterial species [86] .

Beside the nblA, there are four other genes that have been identified as being related to phycobilisome degradation: nblR, nblS, nbl,B, and nblC. The NblR protein controls nblA expression and is critical for survival under stress conditions [87] . It has been suggested that nblR is controlled by nblS, a sensor histidine kinase that has a PAS domain [88]. The NblB protein is necessary for phycobilisome degradation during starvation; but, unlike the NblA protein, the NblB levels are similar prior to, or during, nutrient starvation [ 89]. This protein is partially homologous with PCB lyases, the enzyme involved in chromophore attachment to the PC apo-protein. The most recent addition to the arsenal of proteins involved in the disassembly of the phycobilisome is the nblC protein, which is also required for nblA expression [90] .

The size of proteins belonging to the NblA family is quite small, ranging from 54 to 65 residues with molecular masses of about 7-7.5 kDa. The sequence identity between NblA proteins is relatively poor, ~30%, which is somewhat surprising since the phycobilisome components on which the NblA interacts are highly homologous, >70%. The first crystal structure ([1OJH] of a member of the NblA protein family (from Anabena sp. PCC 7120) was recently determined by X)ray crystallography [91]; this has been followed up with two additional NblA members (from T. vulcanus ([2Q8V] and [2QDO])and S. elongatus sp. 7942;[92]). All structures are extremely similar, exhibiting a helix-turn-helix motif that dimerizes into a flat four)helical bundle. Based on direct measurements between PC and the NblA (both wild-type and mutant forms), Bienert and coworkers suggested a mode of activity whereby the NblA dimer interacts with PC monomers via its amino and carboxyl termini p^) however no proposal as to the method by which the NblA mechanically disassembles the phycobilisome is presented by the authors. Alternative interaction modes are certainly possible, especially between similar helix-turn-helix motifs found in all PBPs, but await further investigation to prove their validity.

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