Anabaena variabilisATCC 29413 showed a constitutive mechanism for fructose uptake which was further enhanced by growing the cells with fructose. The uptake process was energydependent as indicated the inhibitory effect of the uncoupler carbonylcyanidem-chlorophenylhydrazone (CCCP) and the reduction induced by incubating the cells in the dark or in the light with DCMU. Cells adapted to growth on fructose showed increased rates of respiration both in the dark and in the light. The rate of14CO2evolved from radiolabelled fructose was lower in the light than in the dark or in the presence of DCMU. This fact can be partially explained by the photosynthetic reutilization of respiratory14CO2. Modifications in photosynthesis were observed in fructose-grown cells. PS I and PS II activity measured in spheroplasts obtained by lysozyme treatment were enhanced by fructose probably as a way to compensate the lower concentration of chorophyll showed by fructose-grown cells. The photosynthetic affinity for external CO2and the rate of photosynthesis dependent on external inorganic carbon were reduced by fructose.
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