Core mutations of Synechococcus sp. PCC 7002 phycobilisomes: a spectroscopic study

Three cyanobacterial strains harboring mutations affecting phycobilisome (PBS) cores were studied using steady state absorption and fluorescence and time-resolved fluorescence. The apcF mutant, missing beta 18, and the apcDF mutant, missing both alpha APB and beta 18, showed only small spectroscopic differences from the wild-type strain; their PBS emission was blue shifted by 10 nm, whereas their absorption spectra and time-resolved fluorescence kinetics were virtually unchanged. The third mutant studied was the apcE/C186S mutant in which the chromophore-binding cysteine-186 in the LCM99 polypeptide has been substituted with serine. The apcE/C186S mutant contained a modified chromophore which significantly changed the spectroscopic properties of the PBS complex. The apcE/C186S PBS absorbed more than the wild-type strain at 705 nm, and the emission spectrum gave two peaks at 660 nm and 715 nm. The time-resolved kinetics of the apcE/C186S mutant PBS were also significantly altered from those of the wild-type strain.