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Determination of the Rates of Protein Synthesis and Degradation in Lemna minor

^a Biological Sciences, University of East Anglia, Norwich, England

Attempts to measure the rates of synthesis and degradation of protein in plant tissues with isotopes are complicated by the presence of at least two pools of amino acids, only one of which contributes to the synthesis of protein. Direct measurement of the protein precursor pool is thus difficult. This paper shows that one solution to this problem is to assume that the amino-acyl transfer RNA is the strict precursor of protein amino acid. By using labeled methionine, the variation with time of the specific radioactivities of methionine bound to RNA and protein have been examined under two different growth conditions in Lemna minor. From these data rates of flux of methionine into and out of protein may be easily determined.

A second method of determining the rates of protein synthesis and degradation assumes that the specific radioactivity of the methionyl transfer RNA in label chase conditions is effectively zero. Evidence is presented to support this contention, and the rates determined by this method agree with those calculated by the first method.