GLUCOSE UPTAKE BY CYCLOTELLA CRYPTICA: DARK INDUCTION AND LIGHT INACTIVATION OF TRANSPORT SYSTEM1,2

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Abstract
SUMMARY: Glucose transport capacity of C. cryptica increases in an exponential manner over 24 hr after transfer of the cells from light to complete darkness with little simultaneous increase in cell number. The transport system is rapidly inactivated when cells are transferred back to continuous light. Most of the inactivation takes place while there is still little changes in cell number. When grown on a continuous light regime, the capacity for glucose transport per cell depends on the light intensity. At intensities sufficient to saturate photosynthesis the glucose transport system is only about 5% that of dark‐grown cells, while cells grown at intensities close to the light compensation point have about 30% of the capacity of dark‐grown cells. The action spectrum for inactivation of glucose transport is identical to that for photosynthesis.Cells, whether grown under continuous light, in the dark in the presence of glucose, or kept in the dark without glucose, contain high levels of glucokinase and phosphofructokinase. The glucose transport system is highly specific for glucose; only galactose inhibits the uptake of glucose by about 50% when present at 10 times the concentration of glucose. The glucokinase is even more specific for glucose and is not inhibited by galactose. The phosphofructokinase is inhibited by high concentrations of ATP in cells grown under all conditions. cycloheximide inhibits the induction of glucose transport in the dark, but not the inactivation of the system in the light.