Internal transcribed spacer

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The internal transcribed spacer (ITS) is a concept in molecular biology. The spacer is a sequence of RNA in a primary transcript that lies between precursor ribosomal subunits and is removed by splicing when the structural RNA precursor molecule is processed into a ribosome. These sequences are coded by Ribosomal DNA. Eukaryotic organisms have two internal transcribed spacers; ITS-1 is located between the 18S gene and the 5.8S gene, and ITS-2 is located between the 5.8S and the 28S gene. Ribosomal genes and spacers occur in tandem repeats that are thousands of copies long, each separated by what is termed an intergenic spacer (IGS) or non-transcribed spacer (NTS). The ITS region is widely used in taxonomy and molecular phylogenetics.

The ITS region is now perhaps the most widely sequenced DNA region in fungi. It has typically been most useful for molecular systematics at the species level, and even within species (e.g., to identify geographic races). Because of its higher degree of variation than other genic regions of rDNA (for small- and large-subunit rRNA), variation among individual rDNA repeats can sometimes be observed within both the ITS and IGS regions. In addition to the standard ITS1+ITS4 primers used by most labs, several taxon-specific primers have been described that allow selective amplification of fungal sequences (e.g., see Gardes & Bruns 1993 paper describing amplification of basidiomycete ITS sequences from mycorrhiza samples). ITS region is nowadays being used to know the genetic diversity among different strains of bacteria by sequencing the ITS gene.