Potentional consequences of overreduction of the cytoplasm in the cyanobacterium Synechocystis PCC6803.

Abstract

Mechanism, function and capacity of Photosystem 1 (PS1) driven cyclic electron transfer, and newly acquired evidence for redox poise controled trimerization of PS1 will be presented. Of the total light an estimated meager 4% may be processed through PS1 cyclic in chloroplasts, in cyanobacteria this number is flexible and can become raised to over 20% if external stress requires faster turnover of ATP. Evidence for existence of at least three different pathways for PS1 cyclic in Synechocystis PCC6803 will be presented. A NDH1 catalysed route is the major constitutive one, another route proceeds via ferredoxin-quinone reductase, whereas a third (and fourth?) route are inducible. These additional pathways have been identified after shock-exposure of a Synechocystis culture to high salt. Evidence will be presented that the third route relies on increased expression of ferredoxin-NADP+ reductase (FNR). This enzyme which is known as catalyst for the final step of linear photosynthetic electron transfer, likely acts in PS1 cyclic flow in cyanobacteria through an extended aminoterminal domain. The extension is especially present in cyanobacteria, not in the chloroplast homologue of FNR. De novo FNR synthesis serves PS1 cyclic, as the extension provides for membrane attachment. A pest domain prompts cleavage of the extension to arrive at FNR for its classical function. By coincidence we have noticed that reduced conditions during culture of several cyanobacteria (including Synechocystis PCC6803) may give rise to fluorescence emission from PS1 at 755 nm. The signal was till present known in the cyanobacteria Spirulina only.