Photosynthetic microalgae optimize the utilization of inorganic carbon by active uptake and concentration of inorganic carbon (Ci) around the carbon-fixing enzyme Rubisco. This process, mostly induced under limiting carbon conditions is called the Carbon Concentrating Mechanism or CCM. The photosynthetic green alga Chlamydomonas reinhardtii serves as an excellent model organism for the study of eukaryotic CCMs. However, unlike the prokaryotic cyanobacterial CCM, which has a well-characterized Ci uptake system, the Ci uptake and transport system of the C. reinhardtii CCM is not well understood.
Since CO2 is a small neutral molecule, it is believed to be able to passively diffuse into the cell with or without any assistance from membrane bound transport proteins. However, the charged HCO3- ion needs transporters to facilitate its uptake across each membrane barrier. The hydrophobic barriers posed by the plasma membrane, chloroplast envelope and thylakoid membranes in the path of the charged HCO3- ions ultimate destination to the thylakoid lumen has led to the proposed existence of one or more transport proteins at each of these membrane locations.
The roles of the C. reinhardtii NAR gene family, showing sequence homology to the Formate/Nitrite transporter family, were investigated for any changes with respect to the induction of the CCM. NAR1.2 was found to be the most interesting NAR protein with respect to the CCM. NAR1.2 was localized to the chloroplast envelope and is believed to be part of the chloroplastic Ci uptake system.
The previously identified putative Ci transporter, LCI1 was localized to the plasma membrane. The expression of this protein in a LCI1 deficient background showed a significant increase in Ci uptake and Ci affinity of cells even in the absence of a functional CCM. However, the RNAi mediated knockdown of the protein failed to show any growth deficiencies or changes in photosynthetic rates at different pH levels. LCI1 is a transporter that is part of the plasma membrane Ci uptake system.
In the quest for mutations in either previously identified or novel Ci transporters, an insertional mutagenesis project was also undertaken. The sequenced wild-type strain, CC-503, was found to be a natural mutant for the periplasmic carbonic anhydrase, CAH1.
| Identifer | oai:union.ndltd.org:LSU/oai:etd.lsu.edu:etd-07062013-211518 |
| Date | 10 July 2013 |
| Creators | Mukherjee, Bratati |
| Contributors | Griffin, Gregory, Bartlett, Sue G, Longstreth, David J, Larkin, John C, Moroney, James V |
| Publisher | LSU |
| Source Sets | Louisiana State University |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lsu.edu/docs/available/etd-07062013-211518/ |
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