Horizontal gene transfer

Horizontal gene transfer enables bacteria to exchange DNA. The simplest mechanism for gene transfer is called transformation. Transformation is the import and inheritable integration of DNA from the environment. In other words, bacteria take up DNA from their environment. Subsequently, the newly acquired DNA either integrates into the chromosome or forms self-replicating plasmids.

Illustration of bacterial transformation. Bacteria import DNA from the environment and integrate the newly acquired DNA by recombination into the chromosome. Transformation enables bacteria to acquire parts of genes, entire genes, and even operons.

 

Direct visualization of gene transfer. DNA from a bacterium containing an eyfp gene is present in the environment. eyfp encodes for a yellow fluorescent protein (here shown in red) that can be detected using fluorescence microscopy. At t = 0, a single bacterium is present. As time evolves, the bacterium multiplies. Each bacterium can transform with a well-defined probability. At t = 120 min, a single bacterium transforms and becomes fluorescent. Since the gene encoding for eyfp is integrated into the chromosome, the offspring of the initial transformation are also red fluorescent.

 

From a physicist's point of view, the import of DNA into the cell is a very interesting problem. The DNA molecule has a length of several micrometers and forms a random coil in solution. The size of the pore in the cell envelope has a diameter of nanometers. While the DNA threads into the pore, the number of possible conformations of the DNA is reduced and the conformational entropy decreases. Hence, there has to be work done on the molecule to get it into a state associated with smaller entropy. To ensure directed transport of DNA into the cell, the bacterium must invest energy. We are investigating how chemical energy (e.g. binding energy, ATP, proton motive force) drives import of DNA.

From an evolutionary point of view, it is still unclear how the benefit of transformation outweighs its multiple costs. In general, this comes down to the question how recombination / sex speeds up adaptive evolution. One obvious advantage of gene transfer is that bacteria can rapidly acquire adaptive alleles such as genes encoding for antibiotic resistance. Moreover, gene transfer can reduce clonal interference and maladaptation. On the other hand, bacteria must invest energy to synthesize sophisticated machines in order to import DNA. Under certain conditions, gene transfer can slow down adaptive evolution. We are interested in how bacteria optimize the probability of gene transfer for speeding up adaptation.