Studies of light harvesting complexes from purple photosynthetic bacteria

[摘要] In this thesis light harvesting complexes, the LH2 and core complexes, from several different species of purple photosynthetic bacteria have been analysed both functionally and structurally. Purified monomeric core complexes from Rhodopseudomonas (Rps.) palustris have been used to isolate and identify the putative Protein W. This information was then used to create a Protein W deletion mutant. A low-resolution crystal structure of the monomeric core complex from Allochromatium (Alc.) vinosum is presented which suggests that the LH1 complex is a complete ellipse, unlike the core complex from Rps. palustris.It has previously been shown that some species are able to synthesise LH2 complexes that have different NIR absorption spectra. For example, strains 7050 and 7750 of Rps. acidophila can express both the B800-850 and B800-820 LH2 complexes, whilst strain 10050 only expresses the B800-850 LH2 complex despite evidence to suggest that this strain contains multiple LH2 genes (pucBA genes). It is this homogeneity that has made the LH2 complexes from this strain structurally amenable. Here, genomic DNA from Rps. acidophila strain 10050 has been isolated and sequenced using the next generation sequencing (NGS) technique, Illumina sequencing. So far 8 pucBA gene pairs were identified arranged into 2 distinct operons, one containing B800-850 pucBA genes and pucC, the putative Bchl transporter that is essential for efficient LH2 expression. The second operon contains B800-820 pucBA gene pairs only. Analysis of the protein products of the B800-850 type pucBA gene pairs has shown that none of these proteins match the sequence for the LH2 that is expressed by Rps. acidophila strain 10050. The crystal structure of the LH2 complex from the culture of Rps. acidophila used to isolate the genomic DNA was resolved to 2.05 Å from crystals of the LH2 complex. This structure shows that the protein sequence of the LH2 complex has not changed. Hence, not all the pucBA gene pairs have been identified in the genome sequence data. Currently mate-pair sequencing is being completed to fill in the gaps of sequence data and to complete the genome sequence.LH2 complexes contain carotenoid (Car) and Bchl molecules. In this thesis, the energy transfer mechanisms between Car and Bchl molecules have been investigated using 2-dimensional electronic spectroscopy (2DES). This technique splits the emission and excitation events on 2-dimensions, which can make the less populated ‘dark’ states more visible as overlapping peaks can be separated. Car moleucles are not seen as theoretically efficient in photosynthesis. This is due to short life times of the excited state S2. However, the Car used in photosynthesis have conjugated carbon tails with ≥9 π electrons. According to calculations by Tavan and Schulten, these molecules have the propensity to contain additional excited states that lie below the S2 state that can be involved in energy transfer and increase the efficiency of energy transfer between the Car and Bchl molecules. For the first time an intermediate Car electronic state has been directly observed and shown to be involved in energy transfer between the Car and Bchl molecules.

[发布机构] University:University of Glasgow;Department:Institute of Molecular Cell and Systems Biology

[关键词] Photosynthesis, Core complex, Crystallisation, Protein W, Carotenoids