Cytochrome b-559 is an integral protein of the photosystem II (PSII) reaction center from both plants and cyanobacteria. Cytochrome b-559 has the unique structure of a heme crosslinked $alpha$- and $eta$-subunit heterodimer. The stoichiometry of cytochrome b-559 (one or two copies) per the PSII reaction center has been the subject of controversy and the molar ratio of the heme group to the special chlorophyll P-680 has a number of significant implications on our understanding of the functional role of cytochrome b-559, the mechanism of electron donation in PSII, and the stoichiometry of the other redox-active reaction center components. In order to determine the number of the cytochrome b-559 heme in the PSII reaction center of Synechocystis sp. PCC 6803, the $alpha$- and $eta$-subunits are covalently linked by use of the molecular genetic techniques. The resulting psbEF fusion mutant was able to grow photoautotrophically, implying that the PSII complexes are assembled and functional in thylakoids. This result supports the fact that there are two set of cytochrome b-559 in the PSII reaction center of Synechocystis sp. PCC 6803.