Insertion element (also known as an IS, an insertion sequence element, or an IS element) is a short
DNA
Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
sequence that acts as a simple
transposable element
A transposable element (TE), also transposon, or jumping gene, is a type of mobile genetic element, a nucleic acid sequence in DNA that can change its position within a genome.
The discovery of mobile genetic elements earned Barbara McClinto ...
. Insertion sequences have two major characteristics: they are small relative to other transposable elements (generally around 700 to 2500
bp in length) and only code for proteins implicated in the
transposition activity (they are thus different from other
transposons
A transposable element (TE), also transposon, or jumping gene, is a type of mobile genetic element, a nucleic acid sequence in DNA that can change its position within a genome.
The discovery of mobile genetic elements earned Barbara McClinto ...
, which also carry accessory genes such as
antibiotic resistance
Antimicrobial resistance (AMR or AR) occurs when microbes evolve mechanisms that protect them from antimicrobials, which are drugs used to treat infections. This resistance affects all classes of microbes, including bacteria (antibiotic resis ...
genes).
A particular insertion sequence may be named according to the form IS''n'', where ''n'' is a
number
A number is a mathematical object used to count, measure, and label. The most basic examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can be represented in language with number words. More universally, individual numbers can ...
(e.g. IS''1'', IS''2'', IS''3'', IS''10'', IS''50'', IS''911'', IS''26'' etc.); this is not the only naming scheme used, however.
Composition
These proteins are usually the
transposase
A transposase is any of a class of enzymes capable of binding to the end of a transposon and catalysing its movement to another part of a genome, typically by a cut-and-paste mechanism or a replicative mechanism, in a process known as transpositio ...
which catalyses the enzymatic reaction allowing the IS to move, and also one regulatory protein which either stimulates or inhibits the transposition activity. The coding region in an insertion sequence is usually flanked by
inverted repeat
An inverted repeat (or IR) is a single stranded sequence of nucleotides followed downstream by its complementarity (molecular biology), reverse complement. The intervening sequence of nucleotides between the initial sequence and the reverse complem ...
s. For example, the well-known IS''911'' (1250 bp) is flanked by two 36bp inverted repeat extremities and the coding region has two genes partially overlapping ''orfA'' and ''orfAB'', coding the transposase (OrfAB) and a regulatory protein (OrfA).
In addition to occurring autonomously, insertion sequences may also occur as parts of
composite transposon
A composite transposon is similar in function to simple transposons and insertion sequence (IS) elements in that it has protein coding DNA segments flanked by inverted, repeated sequences that can be recognized by transposase enzymes. A composit ...
s. In a composite transposon, two insertion sequences flank one or more accessory genes, such as an antibiotic resistance gene (e.g.
Tn10, Tn''5''). Nevertheless, there exist another sort of transposons, called unit transposons, that do not carry insertion sequences at their extremities (e.g. Tn''7'').
A complex transposon does not rely on flanking insertion sequences for resolvase. The resolvase is part of the tns genome and cuts at flanking inverted repeats.
Evolutionary role
Although insertion sequences are usually discussed in the context of
prokaryotic
A prokaryote (; less commonly spelled procaryote) is a single-celled organism whose cell lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Greek (), meaning 'before', and (), meaning 'nut' ...
genome
A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
s, certain
eukaryotic
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
DNA sequences belonging to the family of Tc1/''mariner'' transposable elements may be considered to be insertion sequences.
Transposition frequency of IS elements is dependent of multiple parameters, including culture growth phase, medium composition, oxygen tension, growth scale, and structural conformation of target sites (e.g.: curvature, presence of certain motifs, DNA composition).
Recombination between genomic IS sites can enable bacteria to adapt to new environments, making IS elements an important mechanism for evolution in bacteria.
See also
*
Mobile genetic elements
Mobile genetic elements (MGEs), sometimes called selfish genetic elements, are a type of genetic material that can move around within a genome, or that can be transferred from one species or replicon to another. MGEs are found in all organisms. In ...
References
* Campbell, Neil A. and Reece, Jane B. (2002). ''Biology'' (6th ed.), pp. 345–346. San Francisco: Benjamin Cummings. .
*
* Prescott, Lansing M.; Harley, John P.; and Klein, Donald A. (2002). ''Microbiology'' (5th ed.), pp. 298–299. New York: McGraw-Hill. .
* Shuler, Michael L. and Kargi, Fikret (2002). ''Bioprocess Engineering: Basic Concepts'' (2nd ed.), p. 220. Upper Saddle River, NJ: Prentice Hall PTR. .
External links
IS Finder
{{Authority control
Molecular biology
Mobile genetic elements