Fig. 1.

Simplified schematic of nitrogen fixation. (A, Upper) Minimal set of genes that can sustain nitrogen fixation in E. coli (9). (Lower) Eukaryotic genes that can or might functionally replace bacterial counterparts. Color schemes correspond to functional properties of the gene products shown in B. (B) Functional features of gene products shown in A and described in the text. Genes and corresponding functions are indicated by matched colors. Green arrows indicate functional replacement by eukaryotic proteins. For the bacterial system, the iron and sulfur needed to assemble all nitrogenase cofactors are supplied by the product of NifU and NifS. Cofactors are shown as ball-and-stick models: yellow (sulfur), orange (iron), black (carbon), red (oxygen). SAM¹ indicates S-adenosyl methionine. Participants of microbial origin expected to be irreplaceable are indicated by thick borders.