Ecological Roles of Arc Signal Transduction System Revealed by Evolutionary Genetics Analysis

The Arc signal transduction system was studied in γ-proteobacteria to determine evolutionary mechanisms and ecological pressures responsible for the plasticity of this system. Phylogenic analysis of the arcA gene encoding the DNA-binding response regulator suggests that the gene has remained under strong purifying selection throughout its history while the arcB gene encoding the sensor histidine kinase has undergone extensive modification in various lineages. The major observed modification occurs in the Alteromonadaceae family, where the Hpt domain of ArcB has been uncoupled from the sensor component in several genera (Shewanella, Colwellia and Idiomarina). This phenomenon appears to be linked to the free-living lifestyles of the Alteromonads. In contrast to the prevailing view of the Arc system existing only in facultative anaerobes, several of the Alteromonad species which encode Arc systems are strict aerobes or aero-tolerant anaerobes, suggesting additional functions for the Arc system beyond facultative anaerobiosis. Molecular clock estimates using linearized phylogenetics trees for the arcA gene place the origins of the system at 600-1300 million years ago, a period roughly corresponding to the rise in atmospheric oxygen levels that occurred in the Precambrian Era. It is hypothesized that the Arc system originated as an early oxidative stress response mechanism in the γ-proteobacteria and was later co-opted for the facultative anaerobic lifestyle.