Optimum temperature and temperature coefficient of protein synthesis in young wheat plants exhibit phenotypical temperature adaptation. In plants grown for 2 days at either chilling (4 C), medium (20 C). or high (36 C) temperature P. typhoides (Rich.) seedlings (Babala grass) were soil grown at 25 C for 10 days under the same light conditions as the wheat and then exposed to 40 and 10 C, respectively, during a 2-day acclimation period. Thirty shoots were used for each in vivo protein synthesis experiment.L. minor was cultivated under sterile conditions in glass jars (15-cm diameter, 8-cm height) in 400 ml medium after Stewart (15). Cultures were diluted and transferred to a fresh medium as soon as the surface was completely covered with plants (7-10 days). Temperature and light conditions were identical with those for wheat. Logarithmically growing cultures were used for temperature adaptation experiments and exposed to 4 C (2 days ), 20 C (2 days), and 36 C (12 h Unless stated otherwise, uniformly labeled L-['4CJleucine was used, the 1 mm solution had a specific activity of either 0.33 ,uCi/ ,imol or 0.166 ,uCi/,umol. The incubation procedure, the separation of the total protein fraction, and the determination of the specific activity (pmol leucine/mg protein) were outlined in detail in an earlier paper (16). In case of P. typhoides, an additional trichloroacetic acid precipitation step was included. The incubation solutions were used repeatedly after sterile filtration (Sartorius membrane filter, type SM 11407, 0.2,um) until they turned brownish.All experiments were run at least in triplicate. Regression statistics were computed, using the facilities of the Rechenzentrum of University. RESULTS AND DISCUSSIONThe validity of the data from incorporation rates of externally supplied, radioactively labeled amino acids is above all dependent upon the following conditions. First, the rate of amino acid uptake into the metabolic pool(s) is not itself the rate-limiting step for the incorporation of label into protein. Second, the swamping of the metabolic pool(s) by application of rather high exogenous concentrations-which are necessary to bring the precursor pool within the shortest possible time into equilibrium with the added labeldoes not per se affect the in vivo rate of protein synthesis. By the following double-labeling experiment, it could be confirmed that the temperature dependence of ['4C]leucine incorporation into the total protein fraction represents the ,uS of in vivo protein synthesis.['4C]NaHCO3 (5 mCi/mmol) and [4,5-3H]
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