Abundance of 4Fe-4S motifs in the genomes of methanogens and other prokaryotes.

The abundance of 4Fe-4S motifs of the form CX2CX2CX3C was analyzed in the open reading frames (ORFs) of 120 prokaryotic genomes. The abundance of ORFs containing the CX2CX2CX3C motif or isORFs correlated (r=0.82) with methanogenesis (p=0.0001), archaea (p=0.0173), anaerobiosis (p<0.0001) and genome size (p<0.0001). Optimal growth temperature (hyperthermophily) did not correlate with the number of isORFs (p=0.6283). Large numbers of CX2CX2CX3C motifs may be associated with unique physiologies: methanogenic archaea contained the greatest number of CX2CX2CX3C motifs found among the prokaryotic groups; however, only about 15% of the motifs were in genes directly involved in methanogenesis. Large numbers of CX2CX2CX3C motifs may also be associated with generalists such as Desulfitobacterium hafniense, which is an anaerobic bacterium containing multiple reductases.