Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Retroposons and transposable elements

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cells may rely upon glycolysis alone for their supply of ATP.

Glycolysis presumably originated in primitive cells early in

the Earth’s history when very little oxygen was present in the

atmosphere. The glycolysis process has been referred to as

anaerobic respiration, although this term is little used today to

avoid confusion.

See alsoBacterial growth and division; Biochemistry

RRestriction enzymesESTRICTION ENZYMES

Restriction enzymesare proteins that are produced by bacte-

riaas a defense mechanism against virusesthat infect the bac-

teria (bacterial phages). Most bacteria have restriction

modification systems that consist of methylases and restriction

enzymes. In such systems a bacteria’s own DNAis modified by

methylation (the addition of a methyl group, CH 3 ) at a specific

location determined by a specific pattern of nucleotide residue

and protected from degradation by specialized enzymes

termed endonucleases.

The names of restriction enzymes are created from the

first letter of the bacterial genus followed by the first two let-

ters of the species plus a Roman numeral if more than one

restriction enzyme has been identified in a particular species.

Thus, the fifth restriction enzyme from E. coliis called EcoRV

(pronounced e, ko, r five). Besides cloning, restriction

enzymes are used in genetic mappingtechniques, linking the

genome directly to a conventional genetic marker.

Any DNA molecule, from viruses to humans, contains

restriction-enzyme target sites purely by chance and, there-

fore, may be cut into defined fragments of size suitable for

cloning. Restriction sites are not relevant to the function of the

organism, nor would they be cut in vivo,because most organ-

isms do not have restriction enzymes.

There are three types of restriction endonucleases in

bacteria. Type I cuts unmodified DNA at a non-specific site

1000 base pairs beyond the recognition site. Type III recog-

nizes a short asymmetric sequence and cuts at a site 24-26

base pairs from the recognition site. Type II recognizes short

DNA of four to eight nucleotides. Type II restriction enzymes

are widely used in molecular biology. Type II restriction

enzymes have two properties useful in recombinant DNA

technology. First, they cut DNA into fragments of a size suit-

able for cloning. Second, many restriction enzymes make

staggered cuts generating single-stranded ends conducive to

the formation of recombinant DNA. Hamilton Smith identi-

fied the first type II restriction enzyme, HindII, in 1970 at

Johns Hopkins University.

Most type II restriction endonucleases cut DNA into

staggered ends. For example, restriction enzyme EcoRI (from

the bacterium Escherichia coli) recognizes the following six-

nucleotide-pair sequence in the DNA of any organism:

5’–GAATTC–3’, 3’–CTTAAG–5’. This type of segment is

called a DNA palindrome, which means that both strands have

the same nucleotide sequence but in antiparallel orientation.

EcoRI cuts in the six-base-pair DNA between the G and the A

nucleotides. This staggered cut leaves a pair of identical single

stranded ends. Some enzymes cut DNA at the same position of

both strands, leaving both ends blunt.

See alsoCell cycle (prokaryotic), genetic regulation of; DNA

(Deoxyribonucleic acid); Gene amplification; Gene; Genetic

code; Genetic identification of microorganisms; Genetic map-

ping; Genetic regulation of eukaryotic cells; Molecular biol-

ogy and molecular genetics

RETROPOSONS AND TRANSPOSABLE

ELEMENTSRetroposons and transposable elements

Transposable elements are relatively long DNAsequences in

prokaryotic and eukaryotic genomes that act as mobile genetic

elements. These elements, which represent a large part of the

genomes of many species transpose by a mechanism that

involves DNA synthesis followed by random integration at a

new target site in the genome.

All transposable elements encode for transposase, the

special enzyme activity that helps in the insertion of trans-

Restriction enzymes ready for use.

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