Biochemical study of a PSI-LHCI complex and molecular study of fcp genes in the diatom Cylindrotheca fusiformis

Abstract

A photosystem I-light-harvesting complex I (PSI-LHCI) was isolated from oxygen-evolving thylakoids of the diatom Cylindrotheca fusiformis. The circular dichroism (CD) spectrum of the complex resembled the photosystem I (PSI) complex of green plants. A single 77K fluorescence emission was observed at 715 nm. The excitation spectrum confirmed that both chlorophyll c and carotenoids were energetically coupled to chlorophyll a. The complex contained PSI reaction center proteins (PsaA/B, the PSI accessory subunit PsaC, and nine light-harvesting complex (LHC) apoproteins including an 18kD and a 17.5kD protein. Photosystem II core polypeptides were not detected by immuno- or silver staining. Taken together, the CD, fluorescence, and protein data indicate that at least nine LHC apoproteins can be specifically associated with PSI in this diatom. Twenty fcp gene sequences that encode fucoxanthin-chlorophyll a/c light-harvesting proteins (FCPs) and three encoded proteins that are similar to a 17.5kD FCP, but it was not possible to conclusively confirm that any of these genes encode proteins associated LHCI.