Abstract:
Phycobilisomes are the major light harvesting complexes for cyanobacteria and
phycocyanin is the primary phycobiliprotein of the phycobilisome rod. The
phycocyanobilin lyases responsible for chromophorylating the phycocyanin p subunit
(CpcB) have been recently identified in the cyanobacterium Synechococcus sp. PCC
7002. Surprisingly, mutants missing the CpcB lyases were nevertheless capable of
producing pigmented phycocyanin. 10K absorbance measurements revealed that the
energy states of the p phycocyanin chromophores were only subtly shifted; however, 77K
steady state fluorescence emission spectroscopy showed excitation energy transfer
involving the targeted chromophores to be highly disrupted. Such evidence suggests that
phycobilin orientation within the binding domain is specifically modified. We
hypothesized that alternate, less specific lyases are able to act on the p binding sites. A
phycocyanin linker-polypeptide deficient mutant was similarly characterized. The light
state transition, a short term adaptation of the photosynthetic light harvesting apparatus
resulting in the redistribution of excitation energy among the photo systems, was shown to
be dominated by the reallocation of phycocyanin-absorbed excitation energy. Treatment
with a high M phosphate buffer effectively prevented the redistribution of both
chlorophyll a- and phycobilisome- absorbed excitation energy, suggesting that the two
effects are not strictly independent. The mutant strains required a larger redistribution of
excitation energy between light states, perhaps to compensate for their loss in
phycobilisome antenna function.
