Abstract
Rubisco small subunits (RbcSs) are encoded by a nuclear multigene family in plants. Five RbcS genes, OsRbcS1, OsRbcS2, OsRbcS3, OsRbcS4, and OsRbcS5, have been identified in rice (Oryza sativa). Among them, the amino acid sequence of OsRbcS1 differs notably from those of other rice RbcSs. Phylogenetic analysis showed that OsRbcS1 is genetically distant from other rice RbcS genes and more closely related to RbcS from a fern and two woody plants. Reverse transcription-PCR and promoter β-glucuronidase analyses revealed that OsRbcS1 was not expressed in leaf blade, a major photosynthetic organ in rice, but was expressed in leaf sheath, culm, anther, and root central cylinder. In leaf blade of transgenic rice overexpressing OsRbcS1 and leaf sheath of nontransgenic rice, OsRbcS1 was incorporated into the Rubisco holoenzyme. Incorporation of OsRbcS1 into Rubisco increased the catalytic turnover rate and Km for CO2 of the enzyme and slightly decreased the specificity for CO2, indicating that the catalytic properties were shifted to those of a high-activity type Rubisco. The CO2 assimilation rate at low CO2 partial pressure was decreased in overexpression lines but was not changed under ambient and high CO2 partial pressure compared with nontransgenic rice. Although the Rubisco content was increased, Rubisco activation state was decreased in overexpression lines. These results indicate that the catalytic properties of Rubisco can be altered by ectopic expression of OsRbcS1, with substantial effects on photosynthetic performance in rice. We believe this is the first demonstration of organ-specific expression of individual members of the RbcS gene family resulting in marked effects on Rubisco catalytic activity.
Highlights
Rubisco small subunits (RbcSs) are encoded by a nuclear multigene family in plants
These results suggest that the catalytic properties of Rubisco are largely determined by the amino acid sequence of Rubisco holoenzyme comprises eight large subunits (RbcLs)
Deduced amino acid sequences of the coding region of the rice RbcS gene family were obtained from Rice Anotation Project Database (RAP-DB; http://rapdb. dna.affrc.go.jp) and aligned within the gene family or with RbcS from other species
Summary
Rubisco small subunits (RbcSs) are encoded by a nuclear multigene family in plants. Five RbcS genes, OsRbcS1, OsRbcS2, OsRbcS3, OsRbcS4, and OsRbcS5, have been identified in rice (Oryza sativa). Other than OsRbcS1, to our knowledge there are no reports, in any plants, of RbcS genes showing such a large diversity of amino acid sequences and expression patterns. It was found that OsRbcS1 functions in encoding the small subunit of Rubisco in particular tissues such as leaf sheath, and the holoenzyme containing OsRbcS1 has altered kinetic properties similar to the high-catalytic-activity Rubisco typically found in C4 plants.
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