Photosynthetic Organisms that Utilize the Phycobilisome

The phycobilisome serves as the major light-harvesting antenna complex of cyanobacteria, rhodophyta (eukaryotic red algae), and cryptophyta. These organisms collectively make up earth' s largest producers of oxygen and absorbers of CO 2. They can be found in environments ranging from oceans to hot- springs (up to ~70°C) to dry deserts and to the Arctic [12]. In all environments they can use light throughout the visible spectrum due to the coverage provided by the phycobili-some and reaction center pigments. The phycobilisome pigments absorb strongly in the region between 550-660 nm, which complements the spectral region covered by chlorophyll a in the blue (soret band) and red regions. Red algae can grow in much deeper water due to the addition of a larger compliment of phycoerythrin (PE), the blue absorbing phycobiliprotein (PBP).

The study of the phycobilisome and its component protein subunits began almost 150 years ago (for a detailed description of the early period of phycobilisome research see [13]). However, the real first structural studies that led to insights on phycobilisome function began in the 1960s and 1970s in the research groups of E. Gantt, A. Glazer, D. Bryant, H. Zuber, and their coworkers ' 14-20]. These original structural studies coupled the visualization by electron microscopy (EM) of the phycobilisome along the thylakoid membranes, with biochemical and genetic studies that associated the phycobilisome with both the bilin-bearing PBPs and the colorless linker proteins. This was followed by additional EM studies on isolated phycobilisomes using negatively stained samples from different organisms. The intricate structure revealed in these micrographs showed that the phycobilisome is assembled by a large number of seemingly similarly sized disks. The fragility of the complex during sample fixation had a notable affect on its shape and integrity, as can be seen by the lack of uniformity between complexes. However, it became quite clear that the phycobilisome contains two main substructures -a set of rod like stacks of disks surrounding a core of two to five close-packed disks '21]' The number of disks in the core was found to be species dependent '22]' but, in all cases, the rods appeared to radiate out from the core. The fashion and architecture of rod attachment to the core is of major functional importance and will be the focus of the final section (Section 11.3 of this review.

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