Proposed pathways for M-, V- and Fe-cluster biosynthesis. Area shaded in gray represents the common portion of the pathway, including the proteins NifU, NifS, NifV and NifB. Area shaded in blue represents the M-cluster specific maturation pathway, and the red shaded area depicts the V- and Fe-cluster specific maturation pathways. Experimentally verified steps are indicated by solid arrows whereas putative assignments are indicated by dotted arrows. See the main text for detailed descriptions.