Molecular cloning and sequence analysis of the cyanobacterial gene for the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase

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Ribulose-1,5-bisphosphate carboxylase/oxygenase consists of large subunits (LS) and small subunits. In plants, the LS is encoded in chloroplast DNA and the small subunit, in nuclear DNA. In cyanobacteria, both subunits are thought to be encoded in chromosomal DNA because of prokaryotes. The gene for the LS of ribulose-1,5-bisphosphate carboxylase/oxygenase from a cyanobacterium, Anacystis nidulans 6301, has been cloned in pBR322 and subjected to sequence analysis. The coding region contains 1,416 base pairs (472 codons). The deduced amino acid sequence of A. nidulans LS protein shows 80% homology with sequences of maize, spinach, and tobacco LS proteins; the nucleotide sequence of A. nidulans LS gene shows 70% homology with sequences of the plant genes. Between A. nidulans LS and the plant LS proteins there is exact sequence homology around the lysine residue to which the activator CO2 binds and around the two lysine residues to which ribulose 1,5-bisphosphate binds. The amino acid sequence where the LS binds to the small subunit is also highly conserved. From comparison of the LS proteins of A. nidulans and the three plants, the rate of amino acid substitution is estimated to be 0.25-0.5 × 10-9 per year per site, which is far below the median value of various types of proteins (1.2 × 10-9 for hemoglobin α). The LS protein is thus a conserved protein.

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