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 compl exes of cyanobacteria. Synechococcus Pbsomes are comprised of phycocyanins (PC) and allophycocyanins (APC). Pbsomes are major light harvesting antenna e and also absorb ultraviolet-B (UV-B) radiation (280-320 nm). Synechococcu s 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 inves tigated the effect of low dose of UV-B on isolated Pbsome of Synechococcus. Our results demonstrate the following alterations. Absorbance at 623 nm in itially showed a sharp decrease with increasing exposure time to UV-B radia tion. The changes in the visible to near ultraviolet absorption and excitat ion ratio indicated a change in chromophore conformation, upon prolonged ex posure of Pbsomes to UV-B radiation. This modification of chromophore confo rmation appeared to be associated with the lo:ls of energy transfer from PC to APC. Circular dichroism spectra in the amide region showed a significan t loss of the a helical content of Pbsomes when exposed for longer duration to UV-B. CD spectra in the visible region revealed a mark:ed decrease in t he rotational strength at 620 hm. Close monitoring of CD signals emanating in the 500 to 700 nm range further revealed that the decrease in the rotati onal strength was closely associated with an initial red shift in the posit ive CD band of Pbsomes when exposed to UV-B for short duration. However, th e peak became constant over prolonged exposure to UV-B radiation and accomp anied a prominent blue shoulder in the positive CD band which suggests the modification and uncoupling of the various phycocyanobilin (PCB) chromophor es of the Synechococcus Pbsomes.