Two evolutionary biologists from the University of Konstanz, Falk Hildebrand and Prof. Axel Meyer PhD, found that the GC content of DNA – the proportion of the bases guanine and cytosine – is related to evolutionary selection. In a cooperative research project carried out with Prof. Dr. Adam Eyre-Walker from Sussex University (England), the researchers investigated the patterns of synonymous polymorphisms using datasets of 149 bacterial species. The results suggest that GC variations do not occur because of neutral background processes as has previously been assumed, but rather are caused by evolutionary selection acting on the entire genome.
"As different life on earth is, there is nevertheless something that is common to all organisms: the genetic information which consists of the four different DNA bases," explained Hildebrand. The genetic alphabet consists of the letters G, A, T, C: the bases guanine, adenine, thymidine and cytosine. The base sequence constitutes the universal "construction plan" of proteins. Normally, the GC content (i.e. the percentage of guanine (G) and cytosine (C)) is used to determine the composition of an organism's genome.
The genomic GC content of a single species is always the same, but the use of GC varies from around 17% in some species to more than 70% in others. "The GC content can therefore be seen as a kind of dialect of the universal language of life," explains Axel Meyer. "In our study, we investigated the forces that affect these genomic dialects." The researchers analysed the GC content of non-coding DNA in a large number of different bacterial genomes. From a purely statistical perspective, it could have been expected that all four letters of the genetic code would occur to the same extent, i.e. 25% each of the DNA bases: G, C, A and T. However, the researchers found that the four letters did not occur to the same extent, and that the bacterial species investigated revealed wide variations in their GC content.
The variation in the use of G and C is usually attributed to differences of static processes that lead to an increase or decrease in the letters G and C during evolution. However, the results obtained by the evolutionary biologists from Konstanz and Sussex suggest that the difference in the GC content is inconsistent with mutation bias. "We believe that there is selection on genomic base composition, which leads to differences in the GC content. For example, bacteria with a high GC content might be better adapted to life in hot springs," explains Axel Meyer. "The possibility to use bioinformatic methods to analyse the huge number of DNA sequences enabled us to show for more than 170 bacterial genomes that selection has led to different GC contents."
The research results obtained by Hildebrand, Meyer and Eyre-Walker were published in the scientific journal PLoS Genetics, along with results obtained by Professor Dr. Dimitri Petrov and Dr. Ruth Hershberg from Standford University who came to the same conclusions, despite using different methods. Prof. Meyer has been awarded several prizes for his scientific achievements, including the "Ernst Mayr Award" from Harvard University and the Academy Award of the Berln-Brandenburg Academy of Sciences.
Professor Axel Meyer, PhDUniversity of KonstanzDepartment of Zoology/Evolutionary BiologyUniversitätsstraße 1078464 KonstanzTel.: +49 (0)7531 / 88-4163E-mail: axel.meyer(at)uni-konstanz.de
Original publication: Falk Hildebrand, Axel Meyer, Adam Eyre-Walker: "Evidence of Selection upon Genomic GC-Content in Bacteria.“ In: PLoS Genetics, September 2010, Volume 6, Issue 9, e1001107.