The aim of this thesis is to investigate possible functions of the Psb31 protein in the cyanobacterium Synechocystis sp. PCC 6803. This protein was originally identified by Yasuhiro Kashino et al. (2002) as part of an active photosystem II (PSII) preparation and is present only in prokaryotic and eukaryotic photosynthetic organisms. The Arabidopsis thaliana homologue has been show to be involved in D1 turnover and PSII dimerization. The expression of the psb31 homologue in Cyanothece sp. ATCC 51142 is co-regulated with photosynthetic genes, in particular PSII genes. A deletion of the psb31 gene does not impair photoautothrophic growth, but in competition experiments the mutant is out-competed under high light stress. This phenotype is minor as when grown separately, the growth rates and physical properties of Δpsb31 and WT are similar. It could be shown that the decreased fitness is caused by a higher sensitivity to reactive oxygen species, in particular to singlet oxygen rather than to hydrogen peroxide. Singlet oxygen is generated as byproduct of the PSII reaction and is thought to inhibit D1 protein synthesis by interfering with the elongation step of translation, but I found that photoinhibition and recovery are not affected in the Δpsb31 mutant. However, singlet oxygen is efficiently quenched by carotenoids and I found, that a truncated Psb31 protein expressed in E.coli binds carotenoids. A polyclonal antibody against Psb31 was generated and I could show that the protein is a membrane protein located mainly in thylakoid membrane but also present in the plasma membrane. It is part of highly active PSII preparations, but only in sub-stoichiometric amounts. These findings lead me to the conclusion, that Psb31 is a PSII associated protein whose function might be to bind partially assembled PSII complexes and prevent the release of singlet oxygen into the cellular environment.