Chloroplast Polyribosomes and Synthesis of Fraction 1 Protein in the Developing Wheat Leaf

Abstract

In a study of the development of the second leaf of seedling wheat, the contents per lamina of fraction 1 protein and total soluble protein increased for 6 days after leaf emergence, and the contents of cytoplasmic and chloroplast rRNA for 5 days. Net synthesis of fraction 1 protein per unit of cyto-plasmic and chloroplast rRNA decreased with time. During the growth phase, incorporation of radioactive amino acids into fraction 1 protein was proportional to the amount of, rather than the rate of increase of, fraction 1 protein. After the maximum content of fraction 1 protein had been reached, incorporation into fraction 1 protein decreased both with time and relatively to other soluble proteins. The content of RNA and the proportion of rRNA found in cytoplasmic and chloroplast polyribosomes also fell. The rate of decrease in incorporation into fraction 1 protein exceeded the rate of decrease in chloroplast polyribosomes. We conclude that chloroplast polyribosomes are active in the turnover of fraction 1 protein large subunit during the growth phase, but are proportionately more active in making other proteins in older leaves.