Investigating the regulation of the cyanobacterial bidirectional hydrogenase on the molecular level is a prerequisite to learn more about the details of its function and to enhance biohydrogen production. Several deletion mutants of histidine kinase genes (hik) of Synechocystis were analysed concerning their hydrogenase activity. hik7 showed no hydrogenase activity at all. It is supposed that the decreased activity is due to an improper processing of the enzyme. Furthermore, the effect of iron deficiency on the bidirectional hydrogenase was investigated on the level of transcription and enzyme activity. Low iron supply elicited low hydrogenase activity but a high promoter activity. Genetic and phenotypic differences have been repeatedly described for different wild-type (WT) strains of Synechocystis. Here, we report the identification of a substrain that has a severely reduced hydrogenase activity compared to the strain used throughout this study. The hydrogenase activity of the substrain was found to be partly complemented by the addition of nickel. Thus, revealing an inhibited nickel uptake into the cell or insertion into the hydrogenase. It emphasizes the importance of the right choice of WT strain for investigations concerning the hydrogen production capacities. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.