Microbiology and Immunology

Episomes, plasmids, insertion sequences, and transposons WORLD OF MICROBIOLOGY AND IMMUNOLOGY

200

•

dant. Because of his work, he was well traveled. His sexual

behavior helped spread the virus to sexual partners all over

North America, and they subsequently passed the virus on to

other partners. Without the techniques and investigative proto-

cols of epidemiology, the source of the AIDS epidemic would

not have been resolved.

Reconstructing the movements of people is especially

important when the outbreak is of an infectious disease. The

occurrence of the disease over time can yield information as to

the source of an outbreak. For example, the appearance of a

few cases at first with the number of cases increasing over

time to a peak is indicative of a natural outbreak. The number

of cases usually begins to subside as the population develops

immunityto the infection (e.g., influenza). However, if a large

number of cases occur in the same area at the same time, the

source of the infection might not be natural. Examples include

a food poisoning or a bioterrorist action.

The ultimate aim of the various steps taken in an epi-

demiological investigation is to prevent infections by the use

of prudent public healthmeasures, rather than having to rely

on reactive steps such as vaccinationto defeat ongoing infec-

tions. Indeed, for some infections (i.e., HIV, hepatitis B and C)

vaccination may not ultimately prove to be as effective as the

identification of the factors that promote the diseases, and

addressing those factors.

See alsoBacteria and bacterial infection; Epidemics and pan-

demics; Laboratory techniques in immunology; Laboratory

techniques in microbiology

EPIDERMAL INFECTIONS• seeSKIN INFECTIONS

EPISOMES, PLASMIDS, INSERTION

SEQUENCESEpisomes, plasmids, insertion sequences, and transposons , AND TRANSPOSONS

Episomes, plasmids, insertion sequences, and transposons are

elements of DNA (deoxyribonucleic acid) that can exist inde-

pendent of the main, or genomic, DNA.

An episome is a non-essential genetic element. In addi-

tion to its independent existence, an episome can also exist as

an integrated part of the host genome of bacteria. It originates

outside the host, in a virus or another bacterium. When inte-

grated, a new copy of the episome will be made as the host

chromosome undergoes replication. As an autonomous unit,

the viral episome genetic material destroys the host cell as it

utilizes the cellular replication machinery to make new copies

of itself. But, when integrated into the bacterial chromosome

they multiply in cell division and are transferred to the daugh-

ter cells. Another type of episome is called the F factor. The F

factor is the best studied of the incompatibility groups that

have the property of conjugation(the transfer of genetic mate-

rial from one bacterial cell to another). The F factor can exist

in three states. F+ is the autonomous, extrachromosomal state.

Hfr (or high frequency recombination) refers to a factor,

which has integrated into the host chromosome. Finally, F, or

F prime, state refers to the factor when it exists outside the

chromosome, but with a section of chromosomal DNA

attached to it. An episome is distinguished from other pieces

of extrachromosomal DNA, such as plasmids, on the basis of

their size. Episomes are large, having a molecular weight of at

least 62 kilobases.

In contrast to episomes, a plasmid exists only as an

independent piece of DNA. It is not capable of integration

with the chromosomal DNA; it carries all the information nec-

essary for its own replication. In order to maintain itself, a

plasmid must divide at the same rate as the host bacterium. A

plasmid is typically smaller than an episome, and exists as a

closed circular piece of double stranded DNA. A plasmid can

be readily distinguished from the chromosomal DNA by the

techniques of gel electrophoresisor cesium chloride buoyant

density gradient centrifugation. In addition to the information

necessary for their replication, a plasmid can carry virtually

any other gene. While not necessary for bacterial survival,

plasmids can convey a selective advantage on the host bac-

terium. For example, some plasmids carry genes encoding

resistance to certain antibiotics. Such plasmids are termed

resistance or R factors. Other traits carried on plasmids

include degradation of complex macromolecules, production

of bacteriocins (molecules that inhibit bacterial growthor kill

the bacteria), resistance to various heavy metals, or disease-

causing factors necessary for infection of animal or plant

hosts. Such traits can then be passed on to other bacteria, as

some (but not all) plasmids also have the ability to promote

transfer of their genetic material, in a process called conjuga-

tion. Conjugation is a one-way event—the DNA is transferred

from one bacterium (the donor) to another bacterium (the

recipient). All plasmids belong to one of the 30 or more

incompatibility groups. The groups determine which plasmids

can co-exist in a bacterial cell and help ensure that the opti-

mum number of copies of each plasmid is maintained.

Plasmids have been exploited in molecular biology

research. The incorporation of genes into plasmids, which

maintain large numbers of copies in a cell (so-called multi-

copy plasmids), allows higher levels of the gene product to

be expressed. Such plasmids are also a good source of DNA

for cloning.

Transposons and insertion sequences are known as

mobile genetic elements. While they can also exist outside of

the chromosome, they prefer and are designed to integrate into

the chromosome following their movement from one cell to

another. The are of interest to researchers for the insight they

provide into basic molecular biology and evolution, as well as

for their use as basic genetic tools. Transposons contain genes

unrelated to the transpositionof the genetic material from one

cell to another. For example, Class 1 transposons encode drug

resistance genes. In contrast, insertion sequences encode only

the functions involved in their insertion into chromosomal

DNA. Both transposons and insertion sequences can induce

changes in chromosomal DNA upon their exiting and inser-

tions, and so can generate mutations.

See also Bacteria; DNA (deoxyribonucleic acid);

Electro-phoresis; Microbial genetics

womi_E 5/6/03 2:12 PM Page 200