Light regulation of phycobilisome structure and gene expression in Spirulina platensis C1 (Arthrospira sp PCC 9438)

Light intensity is the most significant environmental factor influencing th e light harvesting complexes (phycobilisomes) in cyanobacteria. Spirulina p latensis C1, a filamentous cyanobacterium, was grown in batch culture under low (50 mu E m(-2) s(-1)) and high (500 mu E m(-2) s(-1)) photosynthetic p hoton flux density. Phycobilisomes were isolated, purified and analyzed by spectrophotometry, gel electrophoresis and electron microscopy, The absorpt ion spectra in conjunction with the electron micrographs revealed that they are hemidiscoidal with three allophycocyanin cylindrical cores and six phy cocyanin rods radiating from the core. The 33 kDa linker polypeptide normal ly found in low light cells disappeared under high light resulting in small er phycobilisomes. The decrease in the number of phycobilisomes correlated well with the decrease in phycocyanin and allophycocyanin content from cell s shifted from low to high light. The phycobiliprotein transcripts exhibite d different pattern in response to different light intensities. Three speci es of phycocyanin mRNA of sizes 1.4, 1.5 and 3.5 kb and two species of allo phycocyanin of 1.4 and 1.7 kb were found in low light grown cells. However in high light, the 3.5 kb phycocyanin transcript disappeared while the othe rs remained with subtle changes. This observation indicates that the larges t transcript encoding for phycocyanin subunits and linker polypeptides and the phycobilisome structure are modulated by light intensity.