Nucleotide sequence of a cDNA encoding the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase from Aegilops squarrosa

Rubisco (EC 4.1.1.39), the most abundant protein in the world, is a bifunctional enzyme that catalyzes the carboxylation and the oxygenation of ribulose-1,5-bisphosphate (Miziorko and Lorimer, 1983). The holoenzyme is composed of eight LSUs encoded by chloroplast DNA and synthesized in the chloroplast and eight SSUs encoded by nuclear DNA, synthesized on free cytoplasmic polysomes, transported into chloroplasts, and assembled with the LSUs to form the active enzyme. The SSUs are attached to the top and the bottom of the large octamer, whereas the binding sites for ribulose-1,5-bisphosphate activator CO sub(2), and the divalent metal cofactor are all located on the LSUs. The SSU may not be essential for catalysis, but it is evident that it exerts an important role in maximizing the activity of the enzyme. Here we cloned a full-length cDNA for the SSU of Rubisco from the C sub(3) monocot Aegilops squarrosa, which belongs to one of the genera nearest to wheat. An A. squarrosa cDNA library was prepared and screened with a rbcS DNA fragment of rice (a generous gift from Prof. Ray Wu, Cornell University). An 815-bp nucleotide sequence was obtained that includes 525 bp of coding region (from 39-563), 38 bp of 5' untranslated region, and 252 bp of 3' untranslated region. The transit and mature peptide have, respectively, 47 and 128 amino acid residues. Comparison with the coding region of wheat Rubisco (Broglie et al., 1983) revealed 87.5% homology for the mature peptide and 80.8% homology for the transit peptide at the amino acid level.