Integrons Collect Genes for Transposons

Many bacteria that carry multiple drug resistance have emerged since antibiotic use has become widespread. Antibiotic resistance genes are usually carried on plasmids, many of which may be transferred between bacteria. In many cases the antibiotic resistance genes are actually carried within transposons that are inserted into the plasmids.

Novel antibiotic resistance genes often appear first in transposons of the Tn21 family found in gram-negative bacteria. This group of transposons possesses an internal element known as an integron that acts as a gene acquisition and expression system. An integron consists of a recognition region, the attI site, into which a variety of gene cassettes may be integrated, plus a gene encoding the enzyme responsible for insertion, the integrase. The attI site is flanked by two 7 bp sequences that act as recognition sites for the integrase (Fig. 15.25). Two promoters are situated upstream of the variable region. One is for the integrase gene; the other faces into the variable region and drives transcription of whatever gene has been integrated.

Gene cassettes suitable for integration consist of a structural gene lacking its own promoter plus an integrase recognition sequence, the attC site. The attC sites are rather variable, except for the conserved 7 bp sequences at the ends. [The attC sites were originally referred to as "59 base elements" because the first examples discovered were 59 bp long. However, later examples were found that varied in length and internal sequence.] Gene cassettes may exist temporarily as free circular molecules incapable of replication and gene expression or else integrated into the attI site of an integron. The ultimate source of the genes on the gene cassettes is presently obscure. The reason why most known integron cassettes carry genes for antibiotic resistance is presumably due to observer bias—antibiotic resistance is clinically important and therefore noticed more often.

integrase Enzyme that integrates one segment of DNA into another DNA molecule at a specific site integron Genetic element consisting of an integration site plus a gene encoding an integrase

Conjugative transposons combine the ability to move by transposition and to move from one bacterial cell to another.

Integrons accumulate genes by integration of DNA modules flanked by recognition sequences.

Integrons Collect Genes for Transposons 421

Donor Recipient

CELL CELL

Conjugation

Insertion into recipient chromosome

FIGURE 15.24 Conjugative Transposon

Conjugative transposons move from donor cell to recipient cell during bacterial conjugation. The transposon moves in a precise manner, leaving the donor DNA intact and integrating into the recipient DNA without duplicating the target sequence.

FIGURE 15.24 Conjugative Transposon

Conjugative transposons move from donor cell to recipient cell during bacterial conjugation. The transposon moves in a precise manner, leaving the donor DNA intact and integrating into the recipient DNA without duplicating the target sequence.

Transposons of the Tn21 family are widespread and frequently trade antibiotic resistance genes. The Tn2501 transposon carries no antibiotic resistance genes and appears to have an "empty" integron. It is possible for a single integron to collect multiple genes, each flanked by 7 bp boxes. Other similar integrons are found on various plasmids and transposons of other families.

Although most integrons are located on transposons and/or plasmids, a few are found in the chromosomes of gram-negative bacteria. Such chromosomal integrons may collect several hundred genes and are then known as super-integrons. The best known example is on the second chromosome of Vibrio cholerae (causative agent of cholera) and has collected approximately 200 genes, mostly of unknown function and unknown origin.

FIGURE 15.25 Integrons Collect Antibiotic Resistance Genes

The structure of the integron shows two regions. The upstream region has two 7 bp repeat sequences that are recognition sites for integrase. The downstream region contains the gene for integrase. Expression of the integrase gene causes the capture of various other genes, most noticeably antibiotic resistance genes. Once integrated, these captured genes are expressed from a promoter within the integrase gene.

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