The Glycolate Pathway and Photosynthetic Competence in Euglena

Abstract

The development of glycolate pathway enzymes has been determined in relation to photosynthetic competence during the regreening of Euglena cultures. Phosphoglycolate phosphatase and glycolate dehydrogenase rapidly reached maximal levels of activity but the complete development of ribulose 1, 5-diphosphate carboxylase and concomitant photosynthetic carbon dioxide fixation were not attained until 72 hours of illumination. Specific inhibitors of protein synthesis showed that the formation of ribulose 1, 5-diphosphate carboxylase in both division-synchronized and regreening cultures was prevented by both cycloheximide and D-threo-chloramphenicol, whereas phosphoglycolate phosphatase formation was only inhibited by D-threo-chloramphenicol but not by L-threo-chloramphenicol or cycloheximide. Since cycloheximide prevented ribulose diphosphate carboxylase synthesis and photosynthetic carbon dioxide fixation without affecting phosphoglycolate phosphatase synthesis during regreening, it was concluded that photosynthetic competence was not necessary for the development of the glycolate pathway enzymes. The inhibition of phosphoglycolate phosphatase synthesis by D-threo-chloramphenicol but not by L-threo-chloramphenicol or cycloheximide shows that the enzyme was synthesized exclusively on chloroplast ribosomes, whereas protein synthesis on both chloroplast and cytoplasmic ribosomes was required for the formation of ribulose 1, 5-diphosphate carboxylase. Although light is required for the development of both Calvin cycle and glycolate pathway enzymes during regreening it is concluded that the two pathways are not coordinately regulated.Although exogenous glycolate can support the photoheterotrophic growth of a few algae (23,26), in most instances, the glycolate metabolized by the constituent reactions of the glycolate pathway is derived from an intermediate of the Calvin cycle. In Euglena during 14CO2 photosynthesis at certain stages of the cell cycle, glycine, serine, and glycerate are rapidly labeled; the glycerate formed is uniformly labeled even after short periods of photosynthesis, suggesting that it is derived from uniformly labeled glycolate via the reactions of the glycolate pathway (7,8). Conditions that favor the expression of Calvin cycle enzymes and resultant high photosynthetic rates are accompanied by the synthesis of glycolate pathway enzymes. The regreening of bleached Euglena cells in the light gives a concomitant increase in P-glycolate phosphatase and glycolate dehydrogenase activities (6). Similarly, the rapid increase in photosynthetic rate that results on transfer of Chla-*nydomonas mundana from an acetate-containing medium to autotrophic conditions was accompanied by a striking increase in glycolate dehydrogenase activity (21). In cells already capable of high photosynthetic rates, conditions unfavorable for glycolate biosynthesis such as high CO2 concentration result in the repression of P-glycolate phosphatase (27) and glycolate dehydrogenase (9,27).One possible explanation of these results ...

Photorespiration

“…Hence the glycollate pathway enzymes are not dependent on any intermediate from the Calvin cycle for their synthesis and seem to be made on 80 S ribosomes. Other workers have reached similar conclusions, for wheat [53,54] and Euglena [55]. It is not yet clear whether any of the increase in activity of glycollate pathway enzymes in bean leaves is due to activation of pre-existing protein; most of it seems to be due to increased protein synthesis under the control of phytochrome (PFR).…”

Development of Ribulose-1,5-Diphosphate Carboxylase in Castor Bean Cotyledons

“…The activity of this enzyme increases along with other enzymic and structural components of the developing chloroplast, for 3 to 4 days, by which time the cells are photosynthetically competent (4,7). If at any time during the regreening process Euglena cells are transferred back into darkness there is a striking continued synthesis of RuDP Case in the dark (10,11).…”

Ribulose 1,5-Diphosphate Carboxylase Synthesis in Euglena

“…In contrast, cycloheximide specifically blocks protein synthesis by cytoplasmic ribosomes (2). When chloramphenicol at a concentration of 1 mg/ml is added to the suspending medium of Euglena cells, it effectively blocks protein synthesis on 68S chloroplast ribosomes; RuDPCase synthesis during the regreening of dark grown cells was completely inhibited (5,7). This concentration of chloramphenicol has little effect on cell division or viability in Euglena, suggesting that it does not block protein synthesis on cytoplasmic ribosomes.…”

“…This concentration effectively blocks protein synthesis on cytoplasmic ribosomes in Euglena (2) and prevents cell division and Chl synthesis when dark-grown cells are placed in the light (7,25). When this inhibitor is used at lower concentrations the situation is more complex; 2 to 5 Ag/ml actually stimulates chloroplast development (25) although the reasons for this are unclear.…”

“…The activity of this enzyme increases, along with the cellular level of chlorophyll and other enzymic and structural components of the developing chloroplast, for 3 to 4 days, by which time the cells are fully regreened and photosynthetically competent (4,7). In a previous publication, we reported that if at anytime during the regreening process the cells are transferred back into darkness, there is a striking continued synthesis of RuDPCase in the dark (21).…”