The ultrastructure and polypeptide composition of the pyrenoid from "Dunaliella tertiolecta"

Abstract

Toe pyrenoid is a prominent proteinaceous structure found in the stroma of the chloroplast in unicellular and multicellular algae, and contains the majority of the cellular ribulose 1.5 bisphosphate carboxylase/oxygenase (Rubisco), a key enzyme in the Calvin-Benson cycle of photosynthetic C02 assimilation. Pyrenoids from the marine unicellular algae Dunaliella tertiolecta were isolated after the treatment of 1he chloroplast with mercurium chloride. Transmission and scanning electron microscopy showed that tubule-like structures enter into the intrapyrenoidal space. SDS-PAGE of the pyrenoids showed that most of the components consisted of the large (55 kDa) and sma!I subunits ( 15 kDa) of Rubisco. and seve- ral minor polypeptides (52 and 18 kDa). The possible role of the pyrenoid in C02 assimilation is discussed.