Aminoacylation of tRNA and Protein Turnover in Nitrateand Sulphate-Deficient Spirodela polyrhiza

Cultures of Spirodela polyrhiza were maintained in completeHoagland's medium at 25°C in continuous light. Nitrate-and sulphate-deficient plants were cultured in media containing1/20 Hoagland's nitrate and 1/200 Hoagland's sulphate respectively.After 10 days of growth the plants were examined for total aminoacyltRNA levels. Turnover of leucyl-tRNA and rates of protein synthesiswere assessed by pulse feeding [³H]leucine. Control and nutrient-deficientplants had similar levels of tRNA-associated amino acids. Howeverthe amounts of tRNA, expressed on a fresh weight basis, weresignificantly lower in nitrate- and sulphate-deficient plants.Although the specific radioactivities of leucyltRNA were highestin deficient cultures the rate of turnover of this pool wasless than non-deficient control or nitrate- and sulphate-supplementedplants. Calculation of the average rate constants for proteinsynthesis and degradation showed that nitrate deficiency, althoughnot affecting rates of synthesis, supported rates of proteindegradation that were higher than control cultures. ¹ Present address: The Biological Laboratories, Harvard University,16 Divinity Avenue, Cambridge, Massachusetts 02138, U.S.A. (Received March 7, 1983; Accepted August 16, 1983)