This protocol describes how to transform naturally competent Synechocystis sp. PCC 6803.Use this protocol, if you aim at integrating a nucleotide sequence in the Synechocystis chromosome via homologous recombination. Be aware that Synechocystis PCC 6803 strains from different labs vary in their transformation efficiency.The protocol was modified fromGrigorieva, G. & Shestakov, S. (1982). Transformation in the cyanobacterium Synechocystis sp. 6803. FEMS Microbiol Lett 13, 367-370.