Internal transcribed spacer (
ITS) refers to the spacer DNA situated between the small-subunit
ribosomal RNA (rRNA) and large-subunit
rRNA genes in the
chromosome or the corresponding
transcribed region in the
polycistronic rRNA precursor transcript. In
bacteria and
archaea, ITS is located between the
16S and
23S rRNA genes. On the other hand, there are two ITS's in
eukaryotes;
ITS1 is located between
18S and
5.8S rRNA genes, while
ITS2 is between 5.8S and 25S (in plants, or
28S in animals) rRNA genes. ITS1 corresponds to the ITS in bacteria and archaea, while ITS2 originated as an insertion that interrupted the ancestral 23S rRNA gene.