Bacteria have devised sophisticated signaling systems for eliciting a variety of adaptive responses to their environment, which are generally referred to as the "two-component regulatory system." The widespread occurrence of the two-component systems in both prokaryotes and eukaryotes implies that it is a powerful device for a wide variety of adaptive responses of cells to their environment. The two-component signal transducers contain one or more of three conserved and characteristic phosphotransfer signaling domains, named the "transmitter, receiver, and alternative transmitter." The recently determined entire genomic sequence of Synechocystis sp. strain PCC 6803 allowed us to compile systematically a complete list of genes encoding such two-component signal transduction proteins. The results of such an effort, made in this study, revealed that at least 80 ORFs were identified as members of the two-component signal transducers in this single species of cyanobacteria.