Energy transfer between bacterioochlorophyll and the carotenoids in bacterial photosynthesis

Davidson, Edgar (1981) Energy transfer between bacterioochlorophyll and the carotenoids in bacterial photosynthesis. PhD thesis, University of Glasgow.

Abstract

Carotenoids have two major functions in photosynthesis: (1) They act as accessory light-harvesting pigments, absorbing light at wavelengths where-chlorophylls do not absorb and transferring the / absorbed energy to the chlorophylls. (2) They prevent the chlorophylls from sensitizing the destructive 'photodynamic' reaction which occurs in the presence of light and oxygen. In the photosynthetic bacteria, as in plants, the way in which carotenoids porform these functions is not well understood. In whole cells of photosynthetic bacteria the efficiency of energy transfer from carotenoids to the bacteriochlorophyll (BChl) varies from 30 - 90% depending on the specieso. It is not clear whether this is caused by variation in carotenoids type between species or by differences between the relative arrangement of carotenoids and BChl in pigment-protein complexes. During this project I have developed a method for adding carotenoids to the B850 light-harvesting pigment-protein complex from the carotenoidless mutant Rps. sphaeroides, R26. This should allow investigation into the factors which affect the functions of carotenoids. Since the carotenoids bind to identical sites on the complex, a direct comparison can be made between the energy transfer and photoprotective abilities of a range of carotenoids. During the course of this project I characterised the B850 pigment-protein complex from Rps. sphaeroides, R26. Wild-type Rps. sphaeroides contains two types of light-harvesting complex classified by the wavelengths of their near infra red (NIR) absorption spectra. These are called B800 + 850 and B875 since they show absorption bands at 800 and 850, and 875nxii respectively, R26 shows only a single NIR- absorption maximum at approximately 850nm (when R26 was first isolated this band absorbed at 870nm). R26 is usually thought of as containing only one light harvesting complex, B850, which is thought to be a modified B875 type. Using SDS-polyacrylamide gradient gel electrophoresis' and isoelectric focussing I characterised the B850 complex. I found that the two BChls responsible for the 850nm absorption band are attached to two polypeptides, approximate molecular weight 8 and 10 kilodaltons (kD). These polypeptides appeared to be identical to the polypeptides of B800 + 850 from wild-type Rps. sphaeroides, I therefore suggest that the B850 complex of R26 is a B800 + 850 type of complex vrhich lacks the BChl responsible for the 800nin absorption bande. On gradient gels R26 membranes showed the same three light-harvesting complex bands shown by wild-type membranes. This suggests that R26 in current laboratory use contains 2types of light-harvesting complex. I received a culture of the original strain of R26 (absorbing at 870nm). On gradient gels membranes showed only two light-harvesting polypeptides (approximately 8 and 12kD). In wild-type strains these had previously been identified as belonging to the B875 type of light-harvesting complex. I concluded that the R26 strain has gained a B800 + 85O light-harvesting complex (lacking the 800nm BChl) since it was first isolated in 1963. (Abstract shortened by ProQuest.).

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: Richard Cogdell
• Keywords: Plant sciences
• Date of Award: 1981
• Depositing User: Enlighten Team
• Unique ID: glathesis:1981-72597
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 11 Jun 2019 11:06
• Last Modified: 11 Jun 2019 11:06
• URI: https://theses.gla.ac.uk/id/eprint/72597