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 CO
2 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.
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