Identification of the small subunit of ribulose 1,5-bisphosphate carboxylase as a product of wheat leaf cytoplasmic ribosomes.

Polyribosomes from greening wheat seedlings (Triticum vulgaris) were allowed to incorporate [³H]leucine into proteins in the presence of a wheat germ supernatant fraction under conditions permitting the completion and release of polypeptides by cytoplasmic polyribosomes. The released proteins were analyzed by electrophoresis on polyacrylamide gels in the presence of sodium dodecyl sulfate. Discrete proteins, as well as a variety of poorly resolved proteins, were observed to have been labeled. The molecular weight distribution of the labeled proteins correlated well with the distribution of polyribosome size classes present in the samples. Neither the large nor the small subunit of ribulosebisphosphate carboxylase were detected as labeled peaks by this procedure.Immune precipitates formed by the addition of carrier small subunit, detergent, and anti-small subunit serum to the released proteins contained a substantial proportion of nonspecifically precipitated material resembling the population of released proteins, but they also contained two discrete peaks not resolved previously, one having mobility slightly faster than the light chain of immunoglobulin (20,000 daltons) and the other having mobility identical to that of small subunit carrier (ca. 12,000 daltons). Samples containing the latter material were shown to contain labeled tryptic peptides corresponding to those of the small subunit carrier. The results establish that the small subunit of ribulose bisphosphate carboxylase is the product of a small proportion of cytoplasmic polyribosomes during greening of etiolated wheat seedlings.