There are well established methods used in genetic engineering to enhance the ability of bacteria to take up plasmmids from their environment264. We, however, will focus on the natural processes associated with the horizontal transfer of DNA molecules from the environment into a cell, or from cell to cell. The first of these processes is known as transformation. It is an active process that involves a number of components, encoded by genes that can be on or off depending upon environmental conditions. Consider a type of bacteria that can import DNA from its environment. If, however, the density of bacteria is low, then there will be little DNA to import, and it may not be worth the effort to express the genes and synthesize the proteins involved in the transformation machinery. In fact, bacteria can sense the density of organisms in their environment using a process called quorum sensing, which we will consider in more detail later. Bacteria use quorum sensing systems to synthesize the DNA uptake system when conditions warrant, apparently by activating a specific transcription factor(see above). When present in a crowded environment, the quorum sensing system turns on the expression of the DNA update system and generate cells competent for transformation.
Here we outline the process in one type of bacteria but a functionally similar mechanisms are used inother bacterial species and Archaea. Double-stranded DNA binds to the bacterial cell’s surface through a variety of DNA receptors. In some cases these receptors bind specific DNA sequences, in others they bind DNA generically (that is any DNA sequence). As shown, Gram negative bacteria have two lipid membranes, an outer one and an inner (plasma) membrane, with a periplasmic space in between. In an ATP-hydrolysis coupled reaction, DNA bound to the exterior surface of the bacterium is moved, through a protein pore through the outer membrane and into the periplasmic space, where it is passed to the DNA channel protein, Here one strand is degraded by a nuclease while the other moves through the channel into the cytoplasm of the cell in a 5’ to 3’ direction. Once inside the cell, the DNA associates with specific single-stranded DNA binding proteins and, by a process known as recombination, is inserted into the host genome265. While the molecular details of this process and functionally similar processes are best addressed elsewhere, what is key is that transformation enables a cell to decide whether or not to take up foreign DNA and to add those DNA sequences to its genome.