R u b i s c o
INTRODUCTION
Ribulose bisphosphate carboxylase, also known as Rubisco, is the most abundant protein on earth, making up majority of the total protein in leaf tissue. Rubisco is used in the Calvin cycle to fix CO2 from the atmosphere to form 3-phosphoglycerate, which can then be used to make much needed glucose for the plant. This enzyme is so prevalent in plants because it acts so slowly and is the primary rate-limiting enzyme of the Calvin Cycle. Additionally, it is non-specific for CO2 and can fix O2 as well. When Rubisco fixes O2 rather than CO2, the products are phosphoglycolate and 3-phosphoglycerate, respectfully.
Diagram courtesy of Dr. Terrie Rife (Accessed 24OCT06)
Rubisco is made up of eight small subunits and eight large subunits (Rife 2006). The small subunits are coded for in the DNA of the plant. The large subunits, however are coded for by cholorplastic DNA. It also requires Mg2+ in the active site to be enzymatically active. Mg2+ better enables CO2 to bind to the active site.
Photo courtesy of: SciCom (Accessed 24OCT06)
Since Rubisco works the help make glucose from energy obtained during photosynthesis, it could be hypothesized that at times when the plant needs more energy, such as the early stages of development, there will be more Rubisco. Furthermore, when the plant does not require as much energy, such as senescence or as the plant ages, the levels of Rubisco should decrease. The levels of Rubisco in plant leaves reaches maximum during the maturation of the leaf and degrades continuously during senescence (Suzuki et al. 2001). Because Rubisco is an essential enzyme, controlling the amount of this enzyme is essential. It is possible that control could exist at all levels: transcriptional, translational and even replication where the number of chloroplasts can limit the amount of large subunit. In this study, only the regulation depending upon the number of chloroplasts was considered. Additionally, regulation of this enzyme is the end effect of a delicate balance of synthesis and degradation. The purpose of this study is to determine if the amount of Rubisco decreases with age. To examine this, we conducted a quantitative PCR (qPCR) analysis on chloroplastic DNA that codes for the large subunit, an sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE) gel, and a Western Blot analysis.
Arabidopsis thaliana, the plant model for this experiment contains a small size genome. Most of its genome had been sequenced in the past .It is also extremely useful in laboratory studies due to its rapid growth. Since previous scientist had already discovered so much about the genome of Arabidopsis thaliana, the experiment result would be easier to interpret.