Long terminal repeat
Encyclopedia
Long terminal repeats are sequences of DNA
that repeat hundreds or thousands of times. They are found in retroviral
DNA
and in retrotransposon
s, flanking functional genes. They are used by viruses to insert their genetic sequences into the host genome
s.
LTR -- PBS -- PSI -- gag -- pol -- env -- LTR
LTR = U3RU5
into an RNA intermediate, followed by reverse transcription into complementary DNA
(cDNA) and ultimately dsDNA (double-stranded DNA) with full LTRs. The LTRs then mediate integration of the retroviral DNA via an LTR specific integrase
into another region of the host chromosome
. The first LTR sequences were derived by A.P. Czernilofsky and J. Shine (Proc. Natl. Acad. Sci, USA, Vol 74, pp 1473-1477, 1977; and Nucleic Acids Res., Vol 8, pp 2967-2984, 1980).
Retroviruses such as Human Immunodeficiency Virus (HIV
) use this basic mechanism.
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
that repeat hundreds or thousands of times. They are found in retroviral
Retrovirus
A retrovirus is an RNA virus that is duplicated in a host cell using the reverse transcriptase enzyme to produce DNA from its RNA genome. The DNA is then incorporated into the host's genome by an integrase enzyme. The virus thereafter replicates as part of the host cell's DNA...
DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
and in retrotransposon
Retrotransposon
Retrotransposons are genetic elements that can amplify themselves in a genome and are ubiquitous components of the DNA of many eukaryotic organisms. They are a subclass of transposon. They are particularly abundant in plants, where they are often a principal component of nuclear DNA...
s, flanking functional genes. They are used by viruses to insert their genetic sequences into the host genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
s.
Example
For example, a retrovirus genome might contain the following features:LTR -- PBS -- PSI -- gag -- pol -- env -- LTR
LTR = U3RU5
- LTR - Long terminal repeats
- PBS - Primer binding sitePrimer binding siteA primer binding site is a region of a nucleotide sequence where an RNA or DNA single-stranded primer binds to start replication. The primer binding site is on one of the two complementary strands of a double-stranded nucleotide polymer, in the strand which is to be copied, or is within a...
- PSI - packaging siteRetroviral Psi packaging elementRetroviral Psi packaging element is a cis-acting RNA element identified in the genomes of the retroviruses Human immunodeficiency virus and Simian immunodeficiency virus...
- gagGroup-specific antigenGroup-specific antigen is the genetic material that codes for the core structural proteins of a retrovirus.It comprises part of the gag-onc fusion protein.-Gag in HIV:...
, polReverse transcriptaseIn the fields of molecular biology and biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcribes single-stranded RNA into single-stranded DNA. It also helps in the formation of a double helix DNA once the RNA has been reverse...
, envCapsidA capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...
- genes that code for proteins used in viral infection and reproduction
Transcription
The LTRs are partially transcribedTranscription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
into an RNA intermediate, followed by reverse transcription into complementary DNA
Complementary DNA
In genetics, complementary DNA is DNA synthesized from a messenger RNA template in a reaction catalyzed by the enzyme reverse transcriptase and the enzyme DNA polymerase. cDNA is often used to clone eukaryotic genes in prokaryotes...
(cDNA) and ultimately dsDNA (double-stranded DNA) with full LTRs. The LTRs then mediate integration of the retroviral DNA via an LTR specific integrase
Integrase
Retroviral integrase is an enzyme produced by a retrovirus that enables its genetic material to be integrated into the DNA of the infected cell...
into another region of the host chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
. The first LTR sequences were derived by A.P. Czernilofsky and J. Shine (Proc. Natl. Acad. Sci, USA, Vol 74, pp 1473-1477, 1977; and Nucleic Acids Res., Vol 8, pp 2967-2984, 1980).
Retroviruses such as Human Immunodeficiency Virus (HIV
HIV
Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
) use this basic mechanism.