Genomic Aspects of Nitrogen Fixation in the Clostridia

Abstract

The genus Clostridium is a diverse collection of rod-shaped, spore-forming, obligately anaerobic bacteria, which do not carry out dissimilatory sulfate reduction and usually stain gram positive in young cultures (Collins et al., 1994). At present, more than 160 named species are included in the traditional genus Clostridium (according to the database maintained at the National Center for Biotechnology Information or NCBI, USA). However, it has been proposed that, on the basis of phenotypic criteria and the results of phylogenetic analyses, the diverse species in the traditional genus Clostridium be rearranged into different genera (Collins et al., 1994). In this proposed rearrangement, the genus Clostridium is reserved for species that belong to the rRNA group I of Johnson and Francis (1975). The redefined genus Clostridium, based on Clostridium butyricum, retains the better known nitrogen-fixing species of the traditional genus Clostridium. The genus Paenibacillus contains nitrogen-fixing species that form spores under anaerobic conditions (Rosado et al., 1997). The former Clostridium durum or Paenibacillus durum, the dominant organism found in a sediment core from the Black Sea, has been reclassified as a member of the species Paenibacillus azotofixans (Rosado et al., 1997). Criteria used in the reclassification include the DNA relatedness at the genome level as measured by the DNA-DNA reassociation technique. Thus, some of the spore-forming, nitrogen-fixing rods do not belong to the genus Clostridium. The genus Clostridium has been closely associated with the advancement of our knowledge about biological nitrogen fixation. Modern biochemical studies on nitrogen fixation became possible when consistently active cell-free extracts were