Low dose UV-B induced modification of chromophore conformation and it's interaction with microenvironment in cyanobacterial phycobilisomes.
Phycobilisomes (Pbsomes) are the supra macromolecular pigment protein complexes of cyanobacteria. Synechococcus Pbsomes are comprised of phycocyanins (PC) and allophycocyanins (APC). Pbsomes are major light harvesting antennae and also absorb ultraviolet-B (UV-B) radiation (280-320 nm). Synechococcus Pbsomes, upon exposure to low dose of UV-B (0.28 mW cm-2) for different time intervals showed profound alteration in their steady state absorption, fluorescence excitation and emission characteristics (Sah et. al. Biochem. Mol. Biol.Int., Vol. 44, No. 2, 245-247). In the present study, we investigated the effect of low dose of UV-B on isolated Pbsome of Synechococcus. Our results demonstrate the following alterations. Absorbance at 623 nm initially showed a sharp decrease with increasing exposure time to UV-B radiation. The changes in the visible to near ultraviolet absorption and excitation ratio indicated a change in chromophore conformation, upon prolonged exposure of Pbsomes to UV-B radiation. This modification of chromophore conformation appeared to be associated with the loss of energy transfer from PC to APC. Circular dichroism spectra in the amide region showed a significant loss of the alpha helical content of Pbsomes when exposed for longer duration to UV-B. CD spectra in the visible region revealed a marked decrease in the rotational strength at 620 nm. Close monitoring of CD signals emanating in the 500 to 700 nm range further revealed that the decrease in the rotational strength was closely associated with an initial red shift in the positive CD band of Pbsomes when exposed to UV-B for short duration. However, the peak became constant over prolonged exposure to UV-B radiation and accompanied a prominent blue shoulder in the positive CD band which suggests the modification and uncoupling of the various phycocyanobilin (PCB) chromophores of the Synechococcus Pbsomes.