Role of thioredoxins in regulating cyanobacterial bioenergetics under changing light conditions

Abstract

Cyanobacteria use intricate regulatory networks to maintain photosynthetic efficiency under changing environmental conditions, such as fluctuating light. Thioredoxins are small, ubiquitous, redox-active proteins that regulate the activity of their target proteins through reversible thiol-disulfide exchange reactions and play an important role in cellular redox signalling networks. However, while more well-studied in plants, the role of specific thioredoxins and their targets in cyanobacteria remain largely unknown. The aim of this study was to investigate the roles of thioredoxins TrxB and TrxQ in regulating bioenergetics in Synechocystis sp. PCC 6803 under changing light conditions. To characterise the roles of TrxB and TrxQ, knockout mutant strains lacking these thioredoxins were analysed and compared with their corresponding wild type. Cell growth was observed under fluctuating light to test if the mutants could acclimate to changing light conditions. To analyse oxygen production and consumption during alternating light intensities, real-time oxygen exchange was monitored by using membrane inlet mass spectrometry. Additionally, proton motive force dynamics were studied using electrochromic shift measurements and the redox state of TrxB under different light conditions was analysed through thiol alkylation and mobility shift assays. ΔtrxB and ΔtrxQ strains were able to maintain growth under fluctuating light, whereas growth of the wild type was inhibited. Chemical disruption of thiol redox regulation resulted in enhanced oxygen consumption during both dark adaptation and high light phases. This could be also seen in ΔtrxB during high light. The increase in oxygen consumption was mainly independent of flavodiiron protein activity, suggesting the involvement of respiratory terminal oxidases. The disruption of thiol exchange reactions also resulted in decreased proton motive force and increased thylakoid conductivity in wild type cells, while ΔtrxB displayed a weaker response. These results indicate that thiol redox regulation contributes to controlling oxygen-consuming pathways and bioenergetic processes in Synechocystis sp. PCC 6803, and that TrxB likely participates in this regulation.