Historical Perspective — Development of nif Genetics and Regulation in Klebsiella pneumoniae -- Genetics of Nitrogen Fixation and Related Aspects of Metabolism in Species of Azotobacter: History and Current Status -- Nitrogen Fixation in the Clostridia -- Regulation of Nitrogen Fixation in Methanogenic Archaea -- Nitrogen Fixation in Heterocyst-Forming Cyanobacteria -- N2 Fixation by Non-Hete…