Abstract:Incorporation of radiolabelled amino acids into proteins ofEuglena gracilisrevealed that the amount of labelled protein depends on the conditions of illumination and temperature of cultivation. Protein synthesis was generally lower under dark conditions except at 37 °C. The largest amounts of labelled protein were measured at 21 °C and decreased at higher and lower temperatures. By separating the labelled proteins of the membraneous cell fraction from subcultures under a range of culture conditions, the synthesis of some specific proteins was found to be light‐ and/or temperature‐dependent. On incubating cells taken at different times during a light/dark cycle and under constant conditions, a circadian rhythm of35S‐methionine‐ as well as35S‐cysteine‐incorporation was detected. Thereby the cells incorporated ten‐times less cysteine than methionine. Protein synthesis always peaked during the last quarter of the daily light phase, confirming the rhythmic rise in total protein. The length of the rhythm period, approximately 24 h, was nearly independent of the applied temperature in the range