CHIMERIC ARABIDOPSIS-THALIANA RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE CONTAINING A PEA SMALL-SUBUNIT PROTEIN IS COMPROMISED IN CARBAMYLATION/

A cDNA of pea (Pisum sativum L.) RbcS 3A, encoding a small subunit pro tein (S) of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), has been expressed in Arabidopsis thaliana under control of the cauli flower mosaic virus 35S promoter, and the transcript and mature S prot ein were detected. Specific antibodies revealed two protein spots for the four Arabidopsis S and one additional spot for pea S. Pea S in chi meric Rubisco amounted to 15 to 18% of all S, as judged by separation on two-dimensional isoelectric focusing/sodium dodecyl sulfate-polyacr ylamide gel electrophoresis gels from partially purified enzyme prepar ations and quantitation of silver-stained protein spots. The chimeric enzyme had 11 +/- 1% fewer carbamylated sites and a 11 +/- 1% lower ca rboxylase activity than wild-type Arabidopsis Rubisco. Whereas pea S e xpression, preprotein transport, and processing and assembly resulted in a stable holoenzyme, the chimeric enzyme was reproducibly catalytic ally less efficient. We suggest that the presence of, on average, one foreign S per holoenzyme is responsible for the altered activity. In a ddition, higher-plant Rubisco, unlike the cyanobacterial enzyme, seems to have evolved species-specific interactions between S and the large subunit protein that are involved in carbamylation of the active site .