Consequences of Decreased Light Harvesting Capability on Photosystem II Function in Synechocystis sp. PCC 6803.

Aparna Nagarajan,Michelle Liberton,Himadri Pakrasi,Lawrence Page

doi:10.3390/life4040903

Aparna Nagarajan, Michelle Liberton + Show 2 more

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https://doi.org/10.3390/life4040903

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Abstract

Cyanobacteria use large pigment-protein complexes called phycobilisomes to harvest light energy primarily for photosystem II (PSII). We used a series of mutants with partial to complete reduction of phycobilisomes to examine the effects of antenna truncation on photosystem function in Synechocystis sp. PCC 6803. The antenna mutants CB, CK, and PAL expressed increasing levels of functional PSII centers to compensate for the loss of phycobilisomes, with a concomitant decrease in photosystem I (PSI). This increased PSII titer led to progressively higher oxygen evolution rates on a per chlorophyll basis. The mutants also exhibited impaired S-state transition profiles for oxygen evolution. Additionally, P700+ re-reduction rates were impacted by antenna reduction. Thus, a decrease in antenna size resulted in overall physiological changes in light harvesting and delivery to PSII as well as changes in downstream electron transfer to PSI.

Highlights

Photosynthesis begins with the capture of a photon by a pigment
Our results indicate that the antenna mutants express increasing levels of functional photosystem II (PSII) to compensate for the loss of phycobilisomes, and that this increased PSII titer leads to progressively higher oxygen evolution rates
Changes in the protein environment around PSII in antenna mutants were evident from the differences in the two PSII peaks (685 nm and 695 nm)

Summary

Introduction

Photosynthetic organisms contain pigment proteins organized into antenna, which are highly diverse in structure among different organisms but in all cases function to greatly improve light harvesting capability. A major light-harvesting antenna is the phycobilisome, which associates with the thylakoid membrane and facilitates the absorption and transfer of light energy to the photosynthetic reaction centers [1]. Recent work has revealed a functional megacomplex containing phycobilisome, PSI, and PSII that is proposed to represent the spatial organization of these complexes in the thylakoid membrane system [3]. Modifications to light harvesting antenna in photosynthetic organisms for the purpose of increasing productivity have been a topic of considerable interest [4,5].

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