Cyanobacteria are promising and, quite literally, green candidates for microbial biocatalysts. Their photoautotrophic metabolism offers a genuinely sustainable production system by depending solely on water, carbon dioxide, and sunlight as their energy source. Additionally, their metabolism equips them with unique advantages for biocatalysis [1]. During photosynthesis (i), they produce high amounts of high-energy cofactor NADPH [2] and (ii) oxygen as a by-product [3]. Despite these significant advantages for biocatalysis, progress within this research field is limited by remaining challenges. One of the biggest challenges is still to obtain sufficient expression levels of recombinantly expressed genes within the cyanobacterial host. One substantial regulatory element here represents the ribosome binding site (RBS). Recent studies showed that in cyanobacteria, the efficiency of an RBS varied significantly among different genetic contexts, making it challenging to select a well-fitting RBS a priori [4]. We developed an RBS library and a screening strategy to tackle this issue. By this, we aim to find the optimal RBS for our enzymes of interest in our cyanobacterial host, to boost their expression and by this their activity in the cyanobacterial strain Synechocystis sp. PCC 6803. As enzymes, we investigate an alcohol dehydrogenase (RR-ADH) [5], a ketoreductase (LfSDR1M50) [6], an enoate reductase (YqjM) [7], and a Baeyer-Villiger monooxygenase (CHMOAcineto) [5]. By comparing the biocatalytic activity of different expression strains, we aim to find high-performing cyanobacterial whole-cell biocatalysts. [1] J. Jodlbauer et al. Trends Biotechnology, 2021, 39, 875–889 [2] K. Köninger et al. Angew. Chemie – Int., 2016, 55, 5582-5585 [3] A. Hoschek et al. Angew. Chemie – Int., 2017, 56, 15146-15149 [4] Thiel, K. et al. Microb. Cell Fact., 2018, 17, 1–12 [5] N. Oberleitner et al. Green Chem., 2017, 19, 367-371 [6] J. Guo et al. ChemCatChem, 2018, 10, 5496-5504 [7] T.B. Fitzpatrick, et al. J. Biol. Chem., 2003, 278, 19891-19897