Effects of ultraviolet-B radiation on phycobilisomes of Synechococcus PCC 7942: alterations in conformation and energy transfer characteristics

Phycobilisomes (PBS), the major light harvesting antenna of the cyanobacterium Synechococcus contain phycocyanin (PC) and allophycocyanin (APC) as major pigment‐protein complexes. PBS also absorb ultraviolet‐B (280‐320 nm) radiation. Exposure of Synechococcus PBS to low dose of UV‐B (∼0.28 mw.cm‐2) for 90 min induced change in absorption, emission and excitation characteristics of PBS and these changes got enhanced after 3 h of exposure. Room temperature excitation and emission spectra clearly indicated uncoupling of energy transfer from PC to APC on exposure to UV‐B. Also, the 77K emission spectra suggested that F682 emission originating from APC decreased by 42% after 3 h of exposure. Circular dichroism (CD) spectra of UV‐B exposed PBS indicated changes (14% decrease) in the α‐helical content after 90 min treatment. SDS‐PAGE analysis indicated degradation of a 75 kDa polypeptide (which appear to be a linker polypeptide) on UV‐B treatment. The degradation of this polypeptide seems to induce changes in pigment‐protein interaction and decoupling of energy transfer within the PBS. Our results for the first time clearly indicate that the PBS of Synechococcus are targets for UV‐B damage.