Localization and Function of Phycobiliproteins in a Chlorophyll A/D Containing Oxygenic Photosynthetic Prokaryote, Acaryochloris Marina

Abstract

The phycobiliproteins (PBP) are the major light-harvesting pigments characteristic of cyanobacteria, red algae and cryptophytes. These pigments fall into four general classes, based upon their long wavelength absorption maxima: allophycocyanin (APC), phycocyanin (PC), phycoerythrin (PE) and phycoerythrocyanin (PEC). In cyanobacteria and red algae, the PBP are aggregated in the form of phycobilisomes (PBS) which have been shown to be associated with photosystem II (PSII) forming a PSII-PBSsupercomplex at the cytoplasmic or stromal side of thylakoid membranes. In cryptophytes, however, the PBP (which can be either PE or PC, having no APC) do not form PBS and are located in the thylakoid lumen. Functionally, a generally accepted pathway of energy transfer in cyanobacteria and red algae is from PE and/or PEC via PC and APC to chlorophyll (chl) a with almost 100% efficiency. In cryptophytes, PBP might transfer excitation energy directly to chi a, or in conjunction with chi c overlaps the chi a absorption spectrum of the reaction centers (1). Acaryochloris marina is the sole oxygenic prokaryote known to date that contains chl d as the predominant photosynthetic pigment and PBP and chi a as minor pigments (2). Earlier studies showed that the native PBP aggregates contain PE, PC and APC, which do not form PBS in vivo, but rather resemble the peripheral rods of PBS of other cyanobacteria and red algae (3). In this communication we address the location of the PBP with regard to thylakoid membranes and on the energy transfer chain from PBP to the photochemical reaction center pigments.