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Table 1 Hypotheses proposed to explain GC content variations in eubacteria

From: On the molecular mechanism of GC content variation among eubacterial genomes

Hypotheses Time Content Reference
UV resistance 1970 Since ultraviolet radiation induces the formation of thyminedimers, higher GC content gives a selective advantage to organisms living in niches that are susceptible to direct and intense sunlight. 16,17
Thermal adaptation 1984 Thermophilic organisms demonstrate a tendency to high GC content because thermostable and thermolabile amino acids are encoded by GC-rich and GC-poor codons respectively. 18, 19
AT to GC mutation 1988 Practically all organisms are subjected to directional mutation pressure and this offers plausible explanations for the intensive GC content heterogeneity among different chromosomal regions of vertebrate genomes. 20
Metabolic resource 1995 Differences in directional nucleotide substitution among lineages of mammals can be explained by changes in metabolic physiology. This relationship is thought to be mediated by the effect of oxygen radicals. 21
Coding sequence length 1996 The longest coding sequences (exons) of vertebrates and genes of prokaryotes are more GC-rich than the shortest ones. 22, 23
Nitrogen-fixation 1998 There is a significantly higher GC content in the nitrogen-fixing members of the genus than in those unable to fix nitrogen. 24
Oxygen requirement 2002 Aerobic prokaryotes display a significant increment in genome GC% in relation to anaerobic ones. 25
Environment pressure 2005 The GC content of complex microbial communities seems to be globally and actively influenced by the environment, such as bacteria in surface water samples having a GC-content median of around 34%, while for soil samples, it is around 61%. 26
Genome size 2006 The relationship between genome size and GC level is valid for aerobic, facultative, and microaerophilic species. 27
DNA polymerase III 2007 According to the dimeric combination of alpha subunits, GC contents of eubacterial genomes are partitioned into three groups with distinct GC content variation spectra: dnaE1 (full-spectrum), dnaE2/dnaE1 (high-GC), and polC/dnaE3 (low-GC). 28