Gene Movement Within the Same Bacterium Transposable Elements

For many years, biologists thought that DNA was an extremely stable molecule and that genes did not move around inside a cell. Since DNA is the most important carrier of genetic information, it seemed logical that any rearrangements of DNA inside a cell would disrupt this genetic information and harm the cell. This, however, is not necessarily true. As already mentioned, special segments of DNA found in both bacteria and archaea as well as eukaryotes can jump to any other location in the genome. Not only does this process of transposition create mutations, it also plays an important role in the movement of genes of antimicrobial resistance. In bacteria, since transposons often code for proteins that confer resistance to antimicrobials, their movement can be very important to humans and animals. Indeed, the formation of R factor plasmids with their multiple antimicrobial-resistance genes probably results from the coming together of transposable elements coding for resistance to different antimicrobial medications.

Chromosome

Transposon

Gramnegative cell

Transposon

Transfer

Recipient cell (Gram-negative)

Transposon in chromosome "jumps" to plasmid

Transfer

Recipient cell (Gram-positive)

Recipient cell (Gram-negative)

Recipient cell (Gram-positive)

Conjugation and plasmid transfer

Figure 8.23 The Movement of a Transposon Throughout a Bacterial

Population Note that transfer can be between bacteria belonging to different genera and between Gram-negative and Gram-positive bacteria.

8.12 Gene Movement Within the Same Bacterium—Transposable Elements 213

Transposons and Transfer of Genes to Unrelated Bacteria

Transposons that code for antimicrobial resistance can move from sites on the chromosome of a cell into plasmids in the same cell. Plasmids are then readily transferred by conjugation to other cells. If the plasmid happens to be a self-transmissible, wide host range plasmid, it can transfer into a number of bacteria unrelated to the donor (figure 8.23). The combination of inserting transposable elements into broad host range plas-mids with their subsequent transfer into unrelated bacteria explains how the same antimicrobial-resistance genes can be found in unrelated bacteria.

Structure of Transposons

What is the structure of transposons, and what gives them the ability to move? Several types of transposons exist, varying in the complexity of their structure. The simplest is termed an insertion sequence (IS). The more complex is called a composite transposon. The insertion sequence (IS), consists of a gene that codes for the enzyme, transposase. This enzyme is required for transposition and is bound on either side by about 15 to 25 base pairs, which have the structure shown in figure 8.24a. The key feature of this sequence of nucleotides is that the sequence in one strand of DNA is the same as in the other strand, only going in the opposite direction (see figure 8.24a). Such a sequence is termed an inverted repeat. The E. coli chromosome contains more than a dozen different ISs, labeled IS1, IS2, and so on. Several copies of each are present in the chromosome. They are frequently on plasmids also. They often are responsible for the recombination that occurs between two replicons within the same cell. This recombination accounts for the formation of an Hfr strain from an F+ strain of E. coli (see figure 8.17). â–  Hfr formation, p. 208

A more complex type of transposable element, called a composite transposon, consists of a gene whose product is easily recognized, such as a gene coding for antimicrobial resistance,

Gene coding for the

Figure 8.24 Transposable Elements (a) Insertion sequence. Note that the

(a) Insertion sequence

Gene coding for the

Figure 8.24 Transposable Elements (a) Insertion sequence. Note that the

Antibiotic -resistance gene
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