14.4: Since McClintock- Transposons in Bacteria, Plants and Animals

Transposons exist everywhere we look in prokaryotes, and they account for much eukaryotic repetitive DNA. Sometimes called “jumping genes,” they can be a large proportion of eukaryotic genomes and include some “jumping genes” that no longer even jump (i.e., transpose). Transposons were also once considered useless; they were described as junk DNA containing selfish genes, whose only purpose was self-replication to copy useless, junk DNA sequences. But considering some new evidence, perhaps not!

As you will see, transposition shares many features with DNA replication, recombination, repair, and even viral infection. As you study how transposons move, keep in the back of your mind that transposition is often triggered by cellular stress. Let’s begin with a look at some bacterial transposons first, and then we’ll look at eukaryotic transposons.

14.4.1 Bacterial Insertion Sequences (IS Elements)

IS elements are the first mobile bacterial elements described. Discovered in the late 1960s, many were identified (IS1, IS2…, IS10, etc.). Some inserted into genes (e.g., in the lac operon), but most did not, likely because there is little noncoding (“extra”) DNA in the compact bacterial genome. Without this extra DNA acting as a buffer against damaging mutations, few bacterial cells would live to tell a tale of transposition! Perhaps it should surprise us that IS elements can be made to transpose in the lab but are generally silent in nature.

CHALLENGE

Transpositions of an IS or other element into or near genes cause deleterious mutations. Why not in the lac operon?

Bacterial IS elements vary in length from about 750 to 1425 bp. One is shown in Figure 14.12.

Figure 14.12: Structure of a bacterial IS element

The entire IS element is flanked by direct repeats (i.e., repeated sequences facing the same direction) derived from the mechanism of transposition insertion. Within the element lie transposase and resolvase genes whose products are necessary for mobility. Flanking these genes are a pair of repeat sequences facing in opposite directions. Because bacteria (and plasmids) only tolerate low copy numbers of IS elements, there are typically less than ten copies and as few as one!

14.4.2 Composite Bacterial Transposons: Tn Elements

If a pair of IS elements should lie close to each other, separated by a short stretch of genomic or plasmid DNA, they can transpose together, carrying the DNA between them as part of a composite transposon, or Tn element. If some of the DNA between IS elements in a Tn element contains antibiotic-resistance genes, its transposition can carry and spread these genes to other DNA in the cell. Tn elements (like IS elements) are present in low copy number. Figure 14.13 illustrates a generic Tn element.

Figure 14.13: Structure of a bacterial Tn element.

Antibiotic resistance genes have the medical community worried; their spread has led to antibiotic-resistant pathogens that cause diseases that are increasingly hard and even impossible to treat. Earlier we saw the genetic “transformation” of streptococcal cells that pick up virulence genes in DNA from dead cells. We routinely transform cells with plasmids as part of recombinant DNA experiments, but bacteria can transfer plasmid DNA between themselves quite naturally. During bacterial conjugation, an F (fertility) plasmid normally transfers DNA between compatible bacterial mating types (review bacterial conjugation in chapter 8). An F plasmid containing a Tn element that harbors an antibiotic-resistance gene can thus be passed from donor to recipient during conjugation. The Tn element can then transpose into to the recipient’s bacterial genome. In this way, transposition is a major pathway for the transfer and spread of antibiotic resistance.

14.4.3 Complex Transposons That Can Act Like Bacteriophage

Bacterial complex transposons also contain other genes in addition to those required for mobility. Some complex transposons resemble bacteriophage, and one type of transposon—the Mu phage—is in fact one! Mu can function either as an infectious phage (which reproduces in an infected cell) or as a transposon in the bacterial genome. Transposon genes in a Mu phage are illustrated in Figure 14.14.

Figure 14.14: Structure of Mu phage, a complex transposon.

After infecting a bacterium, Mu can enter the lytic phase of its life cycle, replicating its DNA, producing new infectious phage “particles,” ultimately releasing these by lysing the host bacterial cell. Alternatively, like other phage, Mu can undergo lysogeny, inserting its DNA into the host-cell chromosome. Integrated copies of Mu can excise and reenter the lytic phase to produce more phages, usually if some environmental stress threatens host bacterial survival. But a third lifestyle choice, transposition, is available to Mu once the phage integrates into the bacterial chromosome. The three lifestyle options for Mu phages are illustrated in the next few pages. Figure 14.15 (below) illustrates the lytic and lysogenic lifestyle options for a bacterial virus, and the next illustration (Figure 14.16) shows the additional lifestyle options of Mu phage; the phage DNA can act as a transposable element while in the lysogenic pathway!

Figure 14.15: Life-cycle options for a bacteriophage: The lytic pathway ends with cell lysis and phage release. In the lysogenic pathway, the phage DNA becomes part of the host-cell chromosome.

Figure 14.16: The transposon (third) option for Mu phage: After lysogeny the phage DNA can replicate as part of the host-cell chromosome and excise to produce new phage, or it can transpose itself to new locations in the host cell chromosomal DNA circle.

246 Bacterial Mobile Elements

As we describe eukaryotic transposons, look for similarities to bacterial IS and Tn elements.