As we mentioned in Key Concept 15.1, transposable elements (or transposons) are segments of DNA that can move from place to place in the genome. Genome sequencing has allowed scientists to study these elements more broadly, and they are now known to be widespread in both prokaryotes and eukaryotes. Prokaryotic transposable elements are often short sequences of 1,000 to 2,000 bp, and they can be found both in chromosomes and in plasmids. A transposable element might be at one location in the genome of one E. coli strain, and at a different location in another strain. The insertion of this movable DNA sequence from elsewhere in the genome into the middle of a protein-coding gene can disrupt that gene (Figure 17.4A). In a prokaryote, the mRNA transcribed from the disrupted gene will contain the extra sequence, and the protein it expresses will be altered and almost certainly nonfunctional. So in prokaryotes transposable elements can produce significant phenotypic effects. Sometimes, two transposable elements located near one another (within a few thousand bp) will transpose together and carry the intervening DNA sequence with them, creating what is called a composite transposon (Figure 17.4B). Genes for antibiotic resistance can be multiplied and transferred between bacteria in this way: composite transposons carrying genes for antibiotic resistance can insert into a plasmid that then moves between bacteria by conjugation (see Figure 12.22).

Question

A retrovirus infects a cell and makes a cDNA copy of its genome. The cDNA is inserted into the host cell by the action of viral integrase. The cDNA is carried along with the host chromosome as the cell divides. Through recombination, the cDNA adds a gene for excision. It also adds adjacent genes and is able to move about the genome.

As you saw in Chapter 15, there are several ways for transposable elements to move around the genome. For example, a transposable element may be replicated, and then the copy can be inserted into another site in the genome (the “copy and paste” mode). Or the element might splice out of one location and move to another location (“cut and paste”). Transposable elements usually carry genes for enzymes such as transposases, which catalyze the reactions needed for transposition. Often the elements are flanked by inverted repeat DNA sequences that are recognized by these enzymes.