Ribulose bisphosphate carboxylase, small chain RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is a bifunctional enzyme that catalyses both the carboxylation and oxygenation of ribulose-1,5-bisphosphate (RuBP) [<cite idref="PUB00000766"/>], thus fixing carbon dioxide as the first step of the Calvin cycle. RuBisCO is the major protein in the stroma of chloroplasts, and in higher plants exists as a complex of 8 large and 8 small subunits. The function of the small subunit is unknown [<cite idref="PUB00004619"/>]. While the large subunit is coded for by a single gene, the small subunit is coded for by several different genes, which are distributed in a tissue specific manner. They are transcriptionally regulated by light receptor phytochrome [<cite idref="PUB00004339"/>], which results in RuBisCO being more abundant during the day when it is required.