Rubisco activity is modulated in plants by an inhibitor and an activator. The inhibitor 2′-carboxy arabinitol 1-phosphate (2CAIP) accumulates in some plants during darkness and binds to the active site of Rubisco (5, 6). 2CAIP is degraded by a specific phosphatase, which presumably allows Rubisco to function during photosynthesis in the light. Rubisco can be severely inhibited by a range of sugar bisphosphates, including substrate analogues. The enzyme Rubisco activase has the ability to relieve the inhibition caused by sugar bisphosphates (7), possibly by interacting with Rubisco and altering the affinity of the enzyme for bisphosphates.
Following import into chloroplasts and removal of the transit peptide, mature S subunits are assembled with chloroplast-synthesized L subunits to give the active L8S8 Rubisco holoenzyme (21, 22). This assembly process requires the assistance of another chloroplast protein (23) now known as chaperonin 60 (cpn60) (24, 25). In fact, studies on the assembly of Rubisco in chloroplasts and bacteria (23, 26, 27) led to the discovery of the molecular chaperone cpn60 and its role in the correct folding of Rubisco and many other proteins (24, 25, 28). Productive folding of Rubisco requires Mg , ATP hydrolysis, and a smaller cochaperonin molecule (29). Cpn60-mediated folding of Rubisco in bacteria uses a cochaperonin oligomer with 10-kDa subunits (24), but the folding of Rubisco in chloroplasts seems to involve a co-chaperonin oligomer with 21-kDa subunits (30). The reason for this larger cochaperonin in chloroplasts and the mechanistic details of the Rubisco assembly process in plants are currently under investigation.
photosynthesis | Importance, Process, & Reactions - …
Plant cells compartmentalize Rubisco in chloroplasts, but the genetic information is shared between chloroplast and nucleus. The rbcL genes are present in chloroplast DNA (13), and their transcription and translation in plastids uses sequences that are similar to those found in prokaryotes (14, 15), to the extent of allowing direct expression when transferred to E. coli (16). The S-subunit genes are located on nuclear chromosomes and have a more complex structural arrangement. The rbcS genes contain introns and are present as small multigene families that are often closely linked (17). Light-induced expression is mediated by both phytochrome and blue light photoreceptors (18), and positive and negative regulatory sequences are located in cis-acting transcriptional control regions. The rbcS promoters also appear to contain nuclear matrix attachment regions (MARs) (19), which may be important for their expression. The highest level of rbcS mRNA is found in leaves, but it is also found in the photosynthetic tissues in stems, petals and pods. S subunits are synthesized on free cytoplasmic polyribosomes as precursor molecules with an ^-terminal transit peptide (20). The S-subunit precursors are imported post-translationally into chloroplasts in a process requiring ATP, and the transit peptide is removed (21, 22).