Abstract
The complete nucleotide sequence of the structural gene for each H- and L-type large subunit (LS) of Rubisco (rbcL), which differ in isoelectric point and are related with the differential in vitro CO2 fixation efficiency of this enzyme, was determined using cloned chloroplast (ct) DNA fragments of common wheat (T. aestivum cv. Chinese Spring; H-type) and Ae. crassa (4x accession; L-type). Two rbcL genes have almost an identical nucleotide sequence in both the coding and noncoding regions; only five base substitutions and two deletions/insertions were detected. The coding region of rbcL gene of T. aestivum consists of 1431 bp that can encode a peptide of 477 amino acids, whereas that of Ae, crassa 4x contains 1428bp, which code for a 476-amino acid peptide. Compared to the L-type LS of Ae. crassa, the H- type LS of T. aestivum is assumed to have two amino acid substitutions (Gln→Lys, Asn→Ser) and one amino acid addition (Lys) to the C terminus. This supposition is consistent with the finding that the isoelectric point of the H-type LS is higher than that of the L-type LS. The replacement of Gln in the L-type LS with Lys in the H-type LS is the most plausible amino acid change that causes a great difference in the specific activity between the Rubisco with the H- and L-type LS.
