NITROGEN-FIXING AEROBIC-BACTERIA HAVE HIGHER GENOMIC GC CONTENT THAN NONFIXING SPECIES WITHIN THE SAME GENUS

The genomic GC contents of both nitrogen-fixing and non-fixing members of eight genera of bacteria are investigated. Analysis by t-tests sho wed that in the two aerobic genera investigated (Aquaspirillum and Vib rio) there is a significantly higher GC content in the nitrogen-fixing members of the genus than in those unable to Fix nitrogen, whilst in anaerobic genera there is either no GC bias, or in the case of two gen era (Rhodospirillum and Clostridium) there is a significantly higher G C content in the non-fixing organisms. This suggests that, in many gen era, directional mutational pressures are different in nitrogen-fixing and non-fixing organisms. These results are discussed in the light of known mechanisms of mutation pressure and their relation to environme ntal variables.