Novel Phycoerythrins in Marine Synechococcus spp. : Characterization and Evolutionary and Ecological Implications

Four clones of the marine, unicellular, cyanobacteria Synechococcus spp., were examined for the spectral and biochemical features of their phycoerythrins (PE) and their photosynthetic characteristics. Two spectral types of PE which are distinct from known PEs were found. One PE type possessed absorption maxima at 500 and 545 nm and a fluorescence emission at 560 nm. Upon denaturation in acid-urea, two chromophore absorption maxima were obtained, one corresponding to phycourobilin (A_max 500 nm) and one at 558 nm, ascribed to a phycoerythrobilin-like chromophore. The ratio of phycoerythrobilin-like to phycourobilin chromophores was 4.9:1.3. This PE possessed two subunits of M_rs of 17.0 and 19.5 kD for the α and β subunits, respectively. The other PE possessed a single symmetrical absorption at 551 nm and a fluorescence emission at 570 nm. This phycobiliprotein showed a single chromophore absorption band (A_max 558 nm) and yielded two polypeptides, an α of 17.5 kD and a β subunit of 20.8 kD. Both PEs showed a (α, β)_n structure. The presence of phycoerythrobilin-like chromophores (A_max 558 nm) appears to be diagnostic of this marine cyanobacterial group. The features of these PEs combined with additional biochemical data, suggest a possible evolutionary link between the PE-containing marine Synechococcus group and the red algal chloroplast. When the Synechococcus clones were grown under low light intensity the PE-containing clones showed higher photosynthetic performance, larger photosynthetic units sizes, reaction center I to II ratios near unity, and steeper initial slopes of photosynthesis versus irradiance curves than a non-PE-containing clone. These findings demonstrate the high photosynthetic efficiency of PE-containing clones in low light environments common to middepth neritic and oceanic habitats.