The harvesting of light is an important, primary event in photosynthesis, allowing for the transfer of energy to the reaction centre. Purple photosynthetic bacteria use two types of antenna complexes (LH1, LH2) to enhance the efficiency of light harvesting and funnelling energy into the photosynthetic reaction centre. Marichromatium (Mcr). purpuratum is an example of a species that can synthesise antenna complexes that have an unusual absorption spectrum. This LH2 complex has a single strong absorption band at approximately 830nm, with a shoulder at 800nm, also known as the B800-B830 LH2 complex This thesis investigates Mcr. purpuratum and the structural basis for the unusual spectrum of the LH2 complex. The LH2 complex was isolated, purified and, after optimising the purification for stability, crystallised. A low-resolution crystal structure of the LH2 complex is presented, which suggests that this LH2 complex is an octamer, similar to Phaeospirillum (Phs.) molischianum. During this thesis the genome sequence of Mcr. purpuratum became available and I was able to show it contained three pucBA operons, which expressed three β and three α polypeptides, respectively. These were all identified in the purified LH2 complex, confirming that it is heterogeneous. Mcr. purpuratum is not able to produce an LH2 complex in the absence of carotenoid biosynthesis. This was shown by chemical inhibition of phytoene desaturase with DPA, however this checmical is also toxic to the cell. In order to overcome this genetic manipulation by knock-out of the CrtI gene was attempted but due to time constraints was not completed
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Structural and functional studies of LH2 complexes having unusual spectroscopic properties
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