Competence and Transformation

Transformation is the process of import and inheritable integration of DNA from the environment. As such, it is believed to be a major driving force for evolution. Competence for transformation is widespread among bacterial species. Recent findings draw a picture of a conserved molecular machine that binds DNA at the cell surface and subsequently transports it through the cell envelope. Within the cytoplasm the DNA is coated by proteins that mediate recombination or self-annealing. The regulatory mechanisms and environmental signals affecting competence are very diverse between different bacterial species. Competence in Bacillus subtilis has become a paradigm for stochastic determination of cell-fate. Quantitative analysis at the single cell level in conjunction with mathematical modelling allowed understanding of induction and decline of competence at the systems level. Currently, the picture is emerging of stochastic differentiation as a fitness trade-off in fluctuating environments. Introduction Transformation is one of three mechanisms of bacterial horizontal gene transfer. The earliest reports concerning transformation date back to the identification of DNA as the genetic material (Avery et al., 1944; Griffith, 1928). Here, we define transformation as the active acquisition of extracellular DNA and the heritable incorporation of its genetic information into the host cell. The word ‘active’ is chosen to stress the fact that DNA import is driven by a cellular machine that utilizes energy for DNA transport. Bacteria expressing such DNA import machines are naturally competent for transformation. Natural competence is a genetically programmed physiological state which is distinct from artificial transformation involving electroporation or heat shock/chemical treatment. Transformation has been studied mainly in five model organisms, namely the Gram-positive species B. subtilis and Streptococcus pneumoniae, and the Gram-negative species Neisseria gonorrhoeae, Haemophilus influenzae and Helicobacter pylori. Although the molecular mechanism of DNA uptake appears to be well conserved amongst different species (with the exception of H. pylori), the mechanisms of genetic regulation of competence and DNA specificity differ strongly. For example, B. subtilis and S. pneumoniae develop competence only during distinct growth phases, whereas N. gonorrhoeae is constitutively competent. Furthermore, the sequence specificity of DNA import differs between species. N. gonorrhoeae and H. influenzae possess a strong preference for importing speciesown DNA by recognizing short specific DNA uptake sequences, which have multiple repeats on their chromosomes. Such preference for a DNA uptake sequence is absent in B. subtilis. So far, ~50 bacterial species have been shown to develop natural competence. However, many more bacterial species may develop competence under presently unidentified conditions (Kovacs et al., 2009), in analogy to Vibrio cholerae, which develops competence only on chitin surfaces (Meibom et al., 2005). Transformation in B. subtilis has served as an important model system in molecular microbiology in at least two respects. First, the regulatory network that controls competence development has been investigated in much detail since the 1960ies. Since the molecular interactions are relatively well