Biochemistry of diatom photosynthetic membranes and pigment-protein complexes

Abstract

Diatoms are an ecologically important group of algae in both marine and freshwater systems, but in spite of their significance little is known about the structure of their photosynthetic apparatus. This is due in part to the lack of a highly purified, oxygen-evolving thylakoid membrane preparation. Since thylakoid membranes purified from diatoms using methods developed for green plants did not evolve oxygen, a new procedure was developed for use with diatoms. An oxygen-evolving thylakoid membrane preparation is crucial for the study of photosynthetic pigment-protein complexes from these algae because the stability of the Photosystem I (PS I) and Photosystem II reaction centers was shown to be greatly reduced in thylakoid preparations that did not retain electron transport activity. As a result of the instability of PS I in some thylakoid preparations, a novel chlorophyll-binding complex was isolated that contained only the PsaA polypeptide. The isolation of this complex should prove useful in elucidating the structure of the PS I reaction center in all plants. Immunological and N-terminal protein sequencing methods were used to identify several photosynthetic proteins in the purified thylakoid preparation. These results provided evidence for posttranslational modification of two light-harvesting polypeptides (LHCPs) as well as of the PsaB subunit of the PS I reaction center core. Posttranslational modification of LHCPs and/or of PsaB has not been observed in green plants. In contrast to green plants, PS I in diatoms has been shown to be located in the inner thylakoid membranes. It was hypothesized that proteolytic processing of the C-terminus of PsaB in diatoms may be necessary for the PS I holocomplex to be present in the inner membranes, and that this processing may be responsible for the instability of PS I in purified diatom thylakoids. The existence of a functional, highly purified, and extensively characterized thylakoid preparation from diatoms will promote our understanding of the photosynthetic apparatus in these algae.