Ribulose bisphosphate carboxylase synthesis in barley leaves: a developmental approach to the question of coordinated subunit synthesis.

The coordination of the synthesis of the large and small subunits of ribulose 1,5-bisphosphate carboxylase (RuBPCase) was studied in young light-grown barley (Hordeum vulgare L. var. UC566) leaves. Since a barley leaf is a continuum of different aged cells with the youngest cells at the base and the oldest at the tip, developmental changes could be investigated by comparing different leaf regions. The rate of total cytoplasmic protein synthesis increased to a maximum before the rate of total organelle protein synthesis. The different positions of the maxima suggested that the synthesis of the small RuBPCase subunit on cytoplasmic ribosomes and the large RuBPCase subunit on chloroplast ribosomes might not be coupled during barley leaf development. However, measurements of the amounts and rates of synthesis of the subunits showed that they were coupled. Although the amounts of the RuBPCase subunits increased from the younger to the older leaf regions, the subunits were present in an equimolar ratio. While the rates of synthesis of both subunits increased to a maximum in a midleaf region and then declined, the ratio of the rates remained constant. That the subunit amounts remained equimolar and the synthetic rates proportional while total RuBPCase synthesis was changing indicated that the synthesis of the subunits was closely coordinated during leaf development. A close coordination was also supported by the kinetics of the inhibition of subunit synthesis in the presence of cycloheximide.