Photoautotrophic production of hydrogen by cyanobacteria is of great public interest in promising a clean renewable energy carrier from nature's most plentiful resources, solar energy and water. Previous investigations in this frame led to protocols that generate only a slight and transient increase in H2 evolution, at the expense of reduction in cell growth. Therefore, as an important milestone in the future engineering of robust cyanobacteria for industrial-level photoproduction of hydrogen we propose to (i) use our long-term (25 years) experience in cyanobacterial genetics to construct robust mutants overproducing the hydrogen-evolving machine with no reduction of cell fitness; (ii) thoroughly analyze these mutants to characterize the metabolic adaptation to high-level production of H2; and (iii) use a high throughput protein-protein interaction assay for the identification of novel key players in H2 evolution. For this purpose, we will use the best-characterized cyanobacterium Synechocystis PCC6803 that (i) grows well in diverse environmental conditions; (ii) possesses a small genome fully sequenced (about 4.0 Mb) easily manipulable with the replicating plasmids we made for constitutive or regulated overproduction of proteins and analysis of their sub-cellular localization.