Figure 1. Schematic representation of the topology of the jelly roll motif in proteins, with individual b-strands of the b-sheet depicted as arrows. The N- and C-termini of the motif are labeled.
"Jumping gene" is another term for "transposable element," a discrete piece of DNA that can move between nonhomologous positions in a genome. Such elements were first discovered by Barbara McClintock in the 1940s during her study of irregular pigment patterns in maize kernels. We now know that such transposable elements are very widespread, being found in virtually all organisms examined.
The lively term "jumping genes" conveys a sense of the dynamic nature of some segments of DNA. This dynamic view contrasts with the view of DNA as a static and unchangeable molecule, being passed from generation to generation without alteration. While it is certainly true that in most organisms DNA is not subject to large-scale change, the identification of "jumping genes" revealed that some segments of DNA could move.
The "jumping" of an element from one site to another is usually mediated by information encoded by the transposon. The transposon encodes a special recombinase, a transposase, that mediates element translocation. This transposase binds to and acts on special sequences at the ends of the element to execute the DNA breakage reactions that separate the element from the donor site and then join it to the insertion site. When a transposable element translocates from one site in the genome to another, it can have considerable effect. Insertion into a host gene by such an element will generally inactivate that gene, resulting in a mutation. Indeed, a considerable fraction of spontaneous mutations in some organisms result from transposable element insertion.
Some transposable elements encode only a transposase and have no "activity" other than their ability to cause mutation. Other elements, however, encode additional genes that can have considerable effect of the host organism. For example, many bacterial elements carry an antibiotic-resistance gene so that bacteria containing this element are resistant to this drug. Thus when such a bacterial element transposes from a plasmid on the chromosome, the antibiotic marker gene becomes covalently linked to the chromosome and the bacterium will continue to be drug-resistant in the absence of the plasmid. Conversely, when a drug-resistance transposon translocates from the chromosome onto plasmids that can move from one cell to another, this drug-resistance gene will be transmitted to many other bacteria. We now appreciate that transposable elements are the basis of the rapid spread of antibiotic resistance determinants, an increasingly widespread clinical problem.
Another important type of transposable elements are viruses in which transposition is used to link the viral chromosome to the genome of the infected cell (1). One important class of transposable viruses that affects humans is the human immunodeficiency virus (HIV) virus that can lead to acquired immune deficiency syndrome (AIDS). When the HIV virus enters a human cell, its DNA is integrated into the host human genome by transposition; thus this viral DNA is now a stable part of the genome. Because the virus has integrated, it is always present in the genome and cannot be lost. Thus strategies for treating HIV infection and AIDS rely upon affecting HIV gene expression and action, rather than trying to eliminate the virus.