The cyanobacterium Synechocystis sp. PCC 6803 is readily amenable to targeted mutagenesis: Foreign DNA is taken up spontaneously, and after uptake DNA can be integrated into the organism's genome by homologous recombination. Using appropriate DNA constructs for transformation, specific genes in the organism can be interrupted, deleted, or replaced by modified gene copies. The organism can grow under a number of different conditions, ranging from photoautotrophic to fully heterotrophic modes, making genetic modifications that alter fundamental processes such as photosynthesis and/or respiration feasible. For example, deletion of photosystem I leads to an obligate (photo)heterotrophic strain in which photosystem Il-generated electrons appear to be consumed by respiratory processes, whereas deletion of photosystem II leads to an obligate (photo)heterotrophic strain in which cyclic electron flow around photosystem I appears to remain active. A major advantage of Synechocystis sp. PCC 6803 is that its entire genome has been sequenced (by S. Tabata and co-workers), opening many avenues to address basic and applied research problems. For example, genes can be introduced, modified or deleted, and hypotheses regarding the function of an open reading frame can be tested by deletion of this open reading frame. Methods to modify genes are numerous. In addition to site-directed mutagenesis, novel molecular genetic approaches including 'targeted random mutagenesis', combinatorial mutagenesis and introduction of hybrid genes have come of age and have proven to be very powerful tools in protein engineering. These approaches have been utilized primarily in this strain to study photosynthesis, but applications of this technology, including pathway engineering, alterations of substrate specificity of enzymes and introduction of tolerance to a variety of stresses, are equally feasible in relation to more applied aims. For optimal utilization of the potential of the Synechocystis sp. PCC 6803 system, however, an increased emphasis toward understanding the biochemistry and molecular physiology of cyanobacteria will also be critically important.
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