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We have studied the inhibitory effect of heterocyclic herbicides simazine, paraquat, pyrazon and amitrole on photosynthetic CO2 fixation and on the level of intermediates of the CO2 assimilation cycle in isolated spinach (Spinacia oleracea) chloroplasts, as well as their in vitro activities on ribulose-1,5-bisphosphate carboxylase and fructose-1,6-bisphosphatase. The half inhibitory concentrations (I50) of CO2 assimilation were about 1 μM for simazine and paraquat. and 10 μM for pyrazon. Amitrole, with an I50 100 μM, gave only a weak inhibition. In the presence of simazine or pyrazon the triose-phosphates phosphoglycerate ratio diminished because of a decrease of the triose-phosphates percentage from 47% to 19%, which means an inhibition of the phosphoglycerate reduction step by a low NADPH synthesis. However, there was not a parallel increase of phosphoglycerate, because of collateral pathways leading to phospho-enolpyruvate, amino acids and other non-identified compounds. Paraquat did not give such a decreased ratio, which could he explained as an inhibition of some step of the Calvin cycle later than triose-phosphates by the H2O2 formed in a Mehler reaction. Amitrole did not show any effect on the pattern of intermediates. Simazine and pyrazon at 10 μM concentration promote a 20-30% activation of ribulose-1,5-bisphosphate carboxylase activity, whereas paraquat, pyrazon and simazine showed an I50 about 100 μM for the inhibition of the photosynthetic fructose-1,6-bisphosphatase. [ABSTRACT FROM AUTHOR]

Upon continuous illumination of dark-grown spinach (Spinacia oleracea L. cv. Winter Giant) seedlings, the thioredoxin f (Td f) content (ELISA) showed a steep rise, which can be evaluated after 3 and 36 h illumination as 3 times and 10 times the dark value, respectively. These figures correspond to 0.03% and 0.1% of total soluble protein, which means a higher biosynthetic rate for Td f compared to the average of total proteins in the earlier steps of plant development. After 40–50 h light the Td f level reached its highest value which remained stable for an additional 40 h and then decreased. Pulse-chase in vivo experiments with [35S]-methionine also showed this sharp increase of Td f in the dark-light transition. From the pattern of decay of [35S]-labelled Td f, a half-life of 7 h was determined for this chloroplast protein. In vitro translation experiments with poly(A)-mRNA isolated from illuminated young spinach seedlings, coupled to a wheat-germ synthesizing system, showed the appearance of a labelled fraction of ca 19 kDa molecular mass, recognizable by a specific Td f antiserum. When intact spinach chloroplasts were added to the translation assay medium, and then illuminated, the 19 kDa band disappeared, with a parallel increase of an internalized 13 kDa labelled polypeptide, also recognized by the Td f antiserum. These results are good evidence for a nuclear-coded synthesis of a Td f precursor, which travels through the chloroplast envelope, leaving the functional protein inside the organelle after the loss of a 6 kDa transit peptide. [ABSTRACT FROM AUTHOR]

In conventional photosystem II preparation high amounts of Cu are found. After fractionation by centrifugation, Cu can be completely removed from photosystem II without affecting either its photosynthetic activity or the composition of its specific proteins. We could demonstrate that the Cu was associated with nuclear contaminants in the starch fraction. Among the contaminants, several histones were identified by specific antisera and by N‐terminal sequencing. In order to obtain homogeneous BBY preparations of PSII a procedure is employed that involves a 10000 g centrifugation step and which eliminates non‐specifically bound metals, nucleic acids and histones with the starch pellet. The resulting starch‐free BBY (BBYs‐), which is free of these nuclear contaminants, is an appropriate preparation for biophysical studies or for those of metal interactions with PSII. [ABSTRACT FROM AUTHOR]