Most of the eukaryotic protein-coding sequences are interrupted by multiple non-coding sequences, introns. mature RNA is formed by the removal of this introns from a primary transcript.Splicing is catalyzed by a multicomponent complex, the spliceosome.
Two categories of introns are known, a common U2 type and a rare U12 type. These two types of introns are removed by distinctspliceosomes.
Splicing of U2-type introns is catalyzed by the U2-dependent (major) spliceosome. The U12-dependent (minor) spliceosome, responsible for the excision of the U12-type introns, isstructurally similar to the U2-type spliceosome.
Over 99% of the eukaryotic introns belong to the U2 class, and the remaining ones comprise the U12 class
The U2 introns appear in all eukaryotes butthe U12 introns were first reported only in vertebrates, insects, cnidarians and plants. However, they were later discovered in fungi and some unicellular eukaryotes, for example Rhizopus oryzae,Phytophthora and Acantamoeba castellanii.So it appears that U12 has an early evolutionary origin.
Although U12 introns are very rare they occur in ribosomal protein genes. The presence of U12introns in ribosomal protein genes may be of significance from a regulatory point of view. There is evidence that splicing of U12 introns is comparatively ineffective and is a rate-limiting step in geneexpression.
It has been shown that the rate of removal of U12 introns from the respective transcripts by the minor spliceosome is several-fold slower than the rate of removal of U2 introns by themajor spliceosome, leading to the hypothesis that U12 introns down-regulate the expression of their host genes
The minor form contains some components common to the major splicesome butit contains other unique components as well.
The major spliceosome includes the U1, U2, U4, U5 and U6 spliceosomal RNAs as well as multiple protein factors. The minor spliceosome, responsible for...