Synthesis of the Small Subunit of Ribulose-1,5-bisphosphate Carboxylase by Soluble Fraction Polyribosomes of Pea Leaves

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Abstract
The products of amino acid incorporation by pea (Pisum sativum L.) leaf soluble fraction polyribosomes in the wheat germ system were examined by 2-dimensional electrophoresis and fluorography. There are 2 isoelectric variants of the small subunit of ribulose bisphosphate carboxylase in this organism, and the more alkaline of these is consistently labeled in the cell-free protein-synthesizing system. The more acid variant is also labeled, but often less extensively. A minority of leaf polyribosomes are recovered from low speed sedimentable fractions. While these appear also to synthesize the small subunits, the patterns of labeling do not indicate a preferential synthesis of these polypeptides by the sedimentable fraction polyribosomes. In the same experiments, labeling of the large subunit spots was sharply below background; these results confirm a cytoplasmic site of synthesis for small subunits of ribulose bisphosphate carboxylase.