II. THE PROKARYOTIC CELL: BACTERIA
B. PROKARYOTIC CELL STRUCTURE
3. Structures Located Within the Cytoplasm
c. Plasmids and Transposons
The overall purpose of this Learning Object is:
1) to learn the chemical makeup and the functions associated with bacterial plasmids and transposons; and
2) to introduce the relationship between bacterial plasmids and transposons and the transfer of antibiotic resistance from one bacterium to another.
LEARNING OBJECTIVES FOR THIS SECTION
In this section on Prokaryotic Cell
Structure we are looking at the various organelles or structures that make up
a bacterium. As mentioned in the introduction to this section, a typical bacterium
usually consists of:
Structures located within the cytoplasm of bacteria include the nucleoid, ribosomes, and in some bacteria, plasmids, endospores, inclusion bodies, and organelles used for photosynthesis. We will now look at plasmids and transposons.
Plasmids (def) and Transposons (def)
In addition to the nucleoid, many bacteria often contain small nonchromosomal DNA molecules called plasmids. Plasmids usually contain between 5 and 100 genes. Plasmids are not essential for normal bacterial growth and bacteria may lose or gain them without harm. They can, however, provide an advantage under certain environmental conditions. For example, under normal environmental growth conditions, bacteria are not usually exposed to antibiotics and having a plasmid coding for an enzyme capable of denaturing a particular antibiotic is of no value. However, if that bacterium finds itself in the body when the particular antibiotic that the plasmid-coded enzyme is able to degrade is being given to treat an infection, the bacterium containing the plasmid is able to survive and grow.
A. Structure and Composition
Plasmids (def) are small molecules of double stranded, helical, non-chromosomal DNA. In most plasmids the two ends of the double-stranded DNA molecule that make up plasmids covalently bond together forming a physical circle. Some plasmids, however, have linear DNA.
Plasmids replicate independently of the host chromosome, but some plasmids, called episomes (def), are able to insert or integrate into the host cell’s chromosome where their replication is then regulated by the chromosome.
Although some plasmids can be transmitted from one bacterium to another by transformation (def) and by generalized transduction (def), the most common mechanism of plasmid transfer is conjugation (def). Plasmids that can be transmitted by cell-to-cell contact are called conjugative plasmids (def). They contain genes coding for proteins involved in both DNA transfer and and the formation of mating pairs.
Plasmids code for synthesis of a few proteins not coded for by the nucleoid. For example, R-plasmids, found in some gram-negative bacteria, often have genes coding for both production of a conjugation pilus (discussed later in this unit) and multiple antibiotic resistance. Through a process called conjugation, the conjugation pilus enables the bacterium to transfer a copy of the R-plasmids to other bacteria, making them also multiple antibiotic resistant and able to produce a conjugation pilus. In addition, some exotoxins, such as the tetanus exotoxin, Escherichia coli enterotoxin, and E.coli shiga toxin discussed later in Unit 2 under Bacterial Pathogenicity, are also coded for by plasmids. Thousands of different plasmids are known to exist.
C. Transposons (transposable elements or "jumping genes" (def)) are small pieces of DNA that encode enzymes that transpose the transposon, that is, move it from one DNA location to another, either on the same molecule of DNA or on a different molecule. Transposons may be found as part of a bacterium's nucleoid (conjugative transposons) or in plasmids and are usually between one and twelve genes long. A transposon contains a number of genes, coding for antibiotic resistance or other traits, flanked at both ends by insertion sequences coding for an enzyme called transpoase. Transpoase is the enzyme that catalyzes the cutting and resealing of the DNA during transposition. Thus, such transposons are able to cut themselves out of a bacterial nucleoid or a plasmid and insert themselves into another nucleoid or plasmid and contribute in the transmission of antibiotic resistance among a population of bacteria.
Plasmids can also acquire a number of different antibiotic resistance genes by means of integrons. Integrons (def) are transposons that can carry multiple gene clusters called gene cassettes that move as a unit from one piece of DNA to another. An enzyme called integrase enables these gene cassettes to integrate and accumulate within the integron. In this way, a number of different antibiotic resistance genes can be transferred as a unit from one bacterium to another.
Plasmids and conjugative transposons are very important in horizontal gene transfer in bacteria. Horizontal gene transfer (def), also known as lateral gene transfer, is a process in which an organism transfers genetic material to another organism that is not its offspring. The ability of Bacteria and Archaea to adapt to new environments as a part of bacterial evolution most frequently results from the acquisition of new genes through horizontal gene transfer rather than by the alteration of gene functions through mutations. (It is estimated that as much as 20% of the genome of Escherichia coli originated from horizontal gene transfer.)
Horizontal gene transfer is able to cause rather large-scale changes in a bacterial genome. For example, certain bacteria contain multiple virulence genes called pathogenicity islands that are located on large, unstable regions of the bacterial genome. These pathogenicity islands can be transmitted to other bacteria by horizontal gene transfer. However, if these transferred genes provide no selective advantage to the bacteria that acquire them, they are usually lost by deletion. In this way the size of the bacterium's genome can remain approximately the same size over time. See horizontal gene transfer later in this unit.
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