Cyanobacteria possess an inducible mechanism which enables them to concentrate inorganic carbon (Ci) within the cells. An inactivation library was used to raise the high-CO Prequiring mutant of Synechococcus PCC 7942, IL-2, impaired in rgy 3 3 transport. Analysis of the relevant genomic DNA detected several modifications, probably due to the single crossover recombination, leading to inactivation of ORF467 (designated ictB) in IL-2. IctB contains 10 trans-membrane regions and is homologous to several transport-related proteins from various organisms. Kinetic analyses of rgy 3 3 uptake in the wild type and IL-2 suggested the presence of two or three rgy 3 3 carriers exhibiting different affinities to rgy 3 3 . z 1998 Federation of European Biochemical Societies.
Cyanobacterial mutants are being used to elucidate the processes involved in their ability to adapt to changes in the ambient concentration of CO,, including the operation of the Ci-concentrating mechanism. The latter enables the cells to grow in the presence of low concentrations of ambient CO, (for recent reviews, see Pierce and
Inactivation of ccmO in Synechococcus sp. strain PCC 7942 resulted in a mutant which possesses aberrant carboxysomes and a normal inorganic carbon uptake capability but a reduced ability to photosynthetically utilize the internal inorganic carbon pool. Consequently, it exhibits low apparent photosynthetic affinity for extracellular inorganic carbon and demands high levels of CO2 for growth.
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