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

The products of amino acid incorporation by pea (Pisum sativum L.) leaf soluble fraction polyribosomes in the wheat germ system were examined by two-dimensional electrophoresis and fluorography.There are two 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.