The final form of horizontal gene transfer is one that involves viruses. The structure and behavior of viruses is a complex topic, the details of which are beyond us here, but it is not unreasonable to consider viruses as nucleic acid transport machines. Viruses are completely dependent for their replication on a host cell, they have no active metabolic processes and so are not really alive in any meaningful sense, although they can certainly be infectious (and can be denatured by various treatments). The simplest viruses contain a nucleic acid genome and a protein-based transport and delivery system.We will consider a typical bacterial virus, known as a bacteriophage or bacteria eater, which uses a double stranded DNA molecule to encode its genetic information. The bacterial virus we consider here, the T4 bacteriophage, looks complex and it is (other viruses are much simpler). T4 phage (short for bacteriophage) have a ~169,000 base pair double-stranded DNA genome that encodes 289 polypeptides267. The assembled virus has an icosahedral protein head that contains its DNA molecule and a tail assembly that recognizes and binds to target cells. Once a suitable host is found (based on tail binding to cellular surface molecules), the tail domain attaches and contracts, like a syringe, punching a hole through the cell’s external wall and plasma membrane. The DNA emerges from the bacteriophage and enters the cytoplasm, infecting the cell. Genes within the phage genome are expressed leading to the replication of the phage DNA molecule and the fragmentation of the host cell’s genome. The phage DNA encodes the proteins of the phage will are assembled of new phage heads. DNA is packed into these heads by a protein-based DNA pump, a pump driven by coupling to an ATP hydrolysis complex268. In the course of packaging virus DNA will, occasionally, make a mistake and package a fragment of host DNA. When such a phage particle infects another cell, it injects that cell with a DNA fragment derived from the previous host. Of course, this mispackaged DNA may not contain the genes the virus needs to make a new virus or to kill the host. The transferred DNA can be inserted into the newly infected host cell genome, with the end result being similar to that discussed previously for transformation and conjugation. DNA from one organism is delivered to another, horizontally rather than vertically.
Because the movement of DNA is so common, particularly in the microbial world, a number of defense mechanisms have evolved. These include the restriction / DNA modification systems used widely for genetic engineering, and the CRISPR-CAS9 system, which enable cells to recognize and destroy foreign DNA.