New Insight into Isoprenoid Biosynthesis in the Cyanobacterium Synechocystis sp. strain PCC 6803
Poliquin, Kelly Ann
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In cyanobacteria, many compounds including chlorophylls, carotenoids, and quinones are synthesized from the isoprenoid precursors isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). These Csub5 compounds are products of the well-studied methylerythritol phosphate (MEP) pathway found in cyanobacteria, plant plastids, and many bacteria. Previous studies suggest that isoprenoid biosynthesis via the MEP pathway in the cyanobacterium Synechocystis sp. strain PCC 6803 are more complex than those proposed for a model bacterium. Most notably, in vitro isoprenoid biosynthesis in Synechocystis is stimulated by compounds of the pentose phosphate cycle (PPC) and not by intermediates of the MEP pathway. Isoprenoid biosynthesis in Synechocystis was therefore further investigated by disrupting sll1556, a gene distantly related to type 2 IPP isomerase genes. This gene is not essential under optimal photosynthetic conditions (20 µmol photons/m²/s). IPP isomerase activity could not be shown for the purified protein. Whereas in vitro PPC substrate stimulated isoprenoid biosynthesis could not be demonstrated in delta sll1556 cell-free extracts, it was restorable upon addition of the recombinant Sll1556 protein. PPC-stimulated isoprenoid biosynthesis results in a progression of isoprenoid production (Csub5 to Csub10 to Csub20) in vitro, although PPC compounds were not found to serve as direct substrates. Isoprenoid synthesis activity was unaffected when LytB, the terminal enzyme of the MEP pathway responsible for the production of both IPP and DMAPP, was immunodepleted from the cell-free extract, suggesting LytB activity is not likely to contribute to the observed in vitro isoprenoid synthesis. The physiological importance of Sll1556 was revealed at high light (200 µmol photons/m²/s). High light stress in the delta sll1556 mutant is evident by slower cell growth, a decrease in chlorophyll and carotenoid content, and in fewer thylakoids per cell. Myxoxanthophyll, but not zeaxanthin, increased in high light cells. The exact function of Sll1556 remains to be elucidated, but the combined results are consistent with a role in isoprenoid biosynthesis that is particularly important under high light stress. The mechanism by which Sll1556 is involved in PPC-stimulated isoprenoid synthesis is discussed, as are future areas of exploration for research.