Cover photograph (Copyright © 2005, American Society for Microbiology. All Rights reserved.): Structural representation of the iron-sulfur flavoprotein (ISF) from Methanosarcina thermophila. Each monomer (colored) of the protein contains a [4Fe-4S] cluster as well as a flavin mononucleotide cofactor. With a minimum distance of 24 , however, these are too far apart to interact directly. In its functional state, the protein forms a stable tetramer (grey), where the flavin moiety of one monomer (colored) is in close contact (<4 ) with the iron-sulfur cluster of a neighboring ISF molecule. (See related article on page 3848.)