The cyanobacterium Synechocystis sp. strain PCC 6803 serves as the model for oxygenic photosynthesis as well as metabolic regulation within this environmentally important group of primary producers. Recently, previously unknown transcripts were identified among them many could encode for small proteins/peptides. Within SPP2002, we will initiate the functional annotation of µ-proteins among Synechocystis. Particularly, we will concentrate on three µ-proteins, among them two have been already verified, for their potential to exhibit regulatory functions on primary metabolism in Synechocystis. Candidate genes were selected by genome-wide co-expression/co-localization analyses, which were done by our cooperation partner Prof. Wolfgang R. Hess (University of Freiburg). Biochemical and physiological analysis using mutants (knock out and selected site-specific mutants abolishing the µ-protein expression) and strains ectopically expressing the µ-proteins will reveal if they are really functionally important. We hypothesize that these µ-proteins could directly or indirectly regulate specific enzymes such as glyceraldehyde 3-phosphate dehydrogenase und aconitate dehydratase and participate in the regulation of primary metabolism. Based on this screening, attempts will be started to define the molecular mechanisms for their action.