General effect of photosynthetic electron transport inhibitors on translation precludes their use for investigating regulation of D1 biosynthesis in Synechococcus sp strain PCC 7942

Both light itself and excitation pressure have been implicated as the envir onmental signal that stimulates interchange of the two forms of the D1 prot ein of photosystem II (PS II) in Synechococcus sp. strain PCC 7942. We soug ht an explanation for conflicting reports regarding the role of photosynthe tic electron transport in regulation of psbA expression and D1 interchange. Inhibitors that block at different points in the photosynthetic electron t ransport chain were administered and the effect on expression of psbAII, wh ich encodes the high-light-induced form II of D1, was examined by measuring changes in transcript levels and in the activities of reporter enzymes. Bo th 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of PSII, an d 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), an inhibitor of the cytochrome b(6)/f complex, prevented high-light-induced increase in bet a-galactosidase activity from a psbAII::lacZ gene fusion when added at a co ncentration that completely inhibits photosynthetic electron transport (1 m u M). The same effect was observed for luciferase activity from transcripti onal and translational fusions of psbAII to the luxAB genes from Vibrio har veyi. DCMU (1 mu M) arrested luciferase expression at low-light levels - th us eliminating the high light response - whereas a sublethal concentration (50 nM), which reduces electron transport by 50%, had intermediate effects on psbAII-driven luciferase activity. However, psbAII transcript levels, mo nitored by northern blot analysis, were not altered by electron transport i nhibitors, either at low-light intensity or following a high-light exposure . The suppressive effect of DCMU on expression of reporter enzymes was not restricted to the high-light response of psbAII-driven reporter systems, bu t was also observed using an isopropyl-(-d)-thiogalactopyranoside (IPTG)-in ducible trc promoter fused to luxAB. This construct only marginally respond ed to IPTG addition when DCMU was present. Thus, blocking electron transpor t in Synechococcus affects the translation machinery in a general way, and the use of electron transport inhibitors is of limited value when focusing on specific redox regulation of D1 protein synthesis or degradation.