IS Insertion Sequence Elements

IS Insertion Sequence Elements

	IS Insertion Sequence Elements A new class of mutations was discovered in E. coil which differed from the usual base substitutions. Frame shift mutations and deletion mutations. These mutations are the result of insertion of large segments (700-1400 base pairs) of DNA into structural or regulatory genes. The genetic elements responsible for these mutations have been called IS (insertion sequence)elements. Multiple copies of is elements are present in the chromosome of E. coli and in the F, F' and R episomes.
They mediate the integration of one segment of genetic information into another, and appear to exist in eukaryote as well as prokaryote cells. The IS elements can be transposed from one site to another, and have a role in excision and integration of episomes and in translocating DNA segments responsible for drug resistance. The IS elements have been numbered IS-1 to IS-5, according to the order in which they were discovered. IS-l is about 800 base pairs (bp) long, IS2 1,300 bp, IS3 1,200 bp and IS4 and IS5 1,400 bp. Each of these sequences can appear at one or more sites of the bacterial genome. About eight copies of IS1 and about five copies of IS2 are found in the E. coli chromosome. The chromosome of Salmonella typhimurium contains IS1 but not IS2.IS2 has been shown to occur in F + plasmid DNA. The IS-elements are simple insertion sequences. They contain no genes unrelated to insertion function.
The R factors of the fi + class consist of two units, the resistance transfer factor (RTF) and the drug resistance determinant (r determinant). The RTF codes the functions required for the transfer of the plasmid, while the determinant carries most of the antibiotic resistance genes. Both units can replicate autonomously if dissociated from one another. A diagramatic representation of the features observed in different R plasmids (Rldrd19, R6, R100-I). Two IS1 elements, both orientated in the same direction, separate the RTF from the r determinant, and mark the boundaries between the two units. .
In R6 and R100-I plasmids, IS2 is located at a position where it contributes to the transfer positive character of the plasmids. Two IS3 elements are found to bracket the tetracycline resistance gene in inverted orientation The IS elements define boundaries between different blocks of genetic information in a replicon, or mark the sites of fusion of two replicons. They act as joints or connections for the modular construction of chromosomes. Reassortment of the different genetic modules can take place with the help of insertion elements, thus offering selection advantage. Moreover, each module can replicate independently of the others.
	The evolutionary, function of the IS elements appears to be to provide sites were DNA from different sources could integrate into a single replicon. Chromosomal rearrangements are brought about by translocation, duplication, inversion or deletion of genetic material. To these modes is now added formation of new chromosomal sequences by IS elements. Deletion of intermittent genetic material. brings nonadjacent regions of the chromosome together, resulting in a Dew chromosomal order. IS1 serves as a generator of deletions.

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