Cyanobacteria, like higher plant chloroplasts, must deal with the demands of diurnal metabolism. Carbohydrate, made in the light by the reductive pentose phosphate cycle, is catabolised at night mainly by the oxidative pentose phosphate pathway. The fedoxin-thioredoxin system is important in regulating these competing pathways in chloroplasts. The vicinal disulphide of thioredoxin is reduced by photosynthetic electron transport in the light [ I ] . This in turn activates several enzymes of the Calvin cycle and deactivates Glucose 6-phosphate dehydrogenase (G6PDH), the enzyme controlling entry of carbohydrates to the oxidative pentose phosphate pathway. At night, in the dark, thioredoxin is largely oxidised reversing the activity state of the two pathways. We recently provided a direct demonstration of lightactivatioddark-deactivation of several Calvin cycle enzymes both in Nosroc sp. Mac [2] and Synechocysris PCC 6803 [3], presumed to reflect regulation by the thioredoxin system. No significant G6PDH activity was detected in the light or dark. Compared with plant chloroplasts, the only cyanobacterial enzyme which differed in its control was the protonmotive ATPase, F,-F,. The y subunit of the cyanobacterial enzyme has already been shown [4,5] to lack a thioredoxin sequence present in higher plants and the presumed lack of lightldark control was confirmed by direct assays [2,3]. In our previous w o k [2,31, cells were grown in continuous light. Nosroc is almost unique amongst the cyanobacteria in that it can grow heterotrophically in complete darkness. We took advantage of this to investigate if enzyme regulation by thioredoxin is flexible and can vary with the mode of growth of the organism. Adaptations of cyanobacterial metabolism were studied in Nosroc sp. Mac cells grown under (a) continuous white light, (b) 14hrs/lOhrs light/dark cycle and (c) complete darkness. Cells were grown in BG11 medium supplemented with 5mM glucose, harvested, and treated with lysozyme [6] to render them osmotically fragile as previously described [2] Dark-adapted lysozyme-treated cells were diluted into an isotonic activation medium (0.33M sorbitol, 3OmM Tricine-KOH, pH 8.0, 5mM MgCI,, 750 U catalaselml), and incubated in the dark or light for 5 min. Aliquots were then taken and enzyme activities measured in a hypotonic medium that caused rapid lysis of the cells. F,-F, and fructose 1,6-bisphosphatase (FBPase) were assayed by the release of phosphate from their respective substrates in a medium containing 5mM MgCI,, 30 mM Tricine-KOH, pH 8.0 and either 2mM ATP or 1mM FBP. G6PDH activity was measured by following NADPH absorbance at 340nm after lysis of the activated cells in a hypotonic medium (5mM MgCI,, 50mM HEPES-KOH pH 7.0) Both dark and light/dark grown cells displayed generally lowered photosynthetic activities compared to light-grown cells. However, the photosynthetic induction period was markedly shorter in the light/dark-grown cells (results not shown). Fig. l(a) shows that F,-F, follows these general trends whereas FBPase activity was actually highest in dark-grown cells. Significantly, FBPase was lightactivated in vivo by the cyanobacterial thioredoxin system in all conditions. By lowering the pH of the assy medium, we were able to detect G6PDH, Fig l(c), which was light-deactivated to some degree in all growth conditions. However, ATPase activity which could not be light-activated regardless of the growth regime. Fig.2 compares the pigment content of cells grown in the differing regimes. Chlorophyll content remains relatively static whilst large changes in phycobiliprotein composition occur that are reminiscent of those seen during complementary chromatic adaptation. Growth in darkness lowers phycoerythrin content and enhances the phycocyanin content. This is under the control of a FIG. 1 Enzymo rctiritios FIG. 2 Pigmontr