Protein Synthesis in Cotyledons of Pisum sativum L: I. Changes in Cell-Free Amino Acid Incorporation Capacity during Seed Development and Maturation.

The changes in protein content of pea cotyledons have been followed during the period from 9 to 33 days after flowering. Initially protein content increased gradually with a rapid period of deposition occurring between days 21 and 27 after flowering. After the 28th day the rate of accumulation of protein declined as the seed dehydrated and matured. At maturity the pea cotyledon contained approximately 25% protein which was divided into albumins and globulins in the ratio of 1:1.4.Analytical data and the incorporation of exogenously supplied (14)C-leucine indicated that albumins were synthesized early in cotyledon development whereas globulin synthesis predominated with increasing maturity.Ribosomal preparations extracted from seeds during the period of rapid protein synthesis contained a high percentage of polysomes. Preparations from older cotyledons with a declining capacity for protein synthesis had few polysomes and an abundance of monosomes. The amino acid-incorporating capacity of ribosomal preparations from cotyledons of varying age was related to the polysomic content. The phenylalanine-incorporating capacity of ribosomal preparations from mature pea seed could be stimulated by the addition of polyuridylic acid. The distribution of polysomes and the in vitro incorporation data suggested that protein synthesis could be partially restricted by the availability of messenger RNA at maturity.However, reciprocal mixing experiments of supernatant and ribosomal fractions from cotyledons of different developmental age indicated that the supernatant fractions have varying capacities to stimulate in vitro amino acid incorporation. Thus the possibility of the regulation of protein synthesis at the translational level was not precluded.