Structure of U12-type Introns and Minor Spliceosome
Although originally referred to as ATAC introns, U2-type introns have GT-AG 5’ and 3’ splice sites while U12-type introns have AT-AC at their 5’ and 3’ ends.
The main determinants for distinguishing U2- and U12-type introns are 5’ splice site and branch site sequences.
Minor spliceosome consists of U11, U12, U4atac, and U6atac, together with U5 and an unknown number of non-snRNP proteins. The U11, U12 and U4atac/U6atac snRNPs are functional analogs of the U1, U2 and U4/U6 snRNPs in major spliceosome. Although the minor U4atac and U6atac snRNAs are functional analogs of U4 and U6, respectively, they share only limited sequence homology (ca. 40%). Furthermore, the sequence of U11 in comparison with U1, as well as U12 compared with U2, are completely unrelated. Despite this fact, the minor U11, U12, U4atac and U6atac snRNAs can be folded into structures similar to U1, U2, U4 and U6, respectively.
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