Minor spliceosome

A notable feature of eukaryotic nuclear pre-mRNA introns is the relatively high level of conservation of the primary sequences of 5' and 3' splice sites over a great range of organisms.

In 1996, Woan-Yuh Tarn and Joan A. Steitz described an in vitro system that splices a pre-mRNA substrate containing an AT–AC intron derived from the human P120 gene.

Psoralen cross-linking confirms the base-pairing interaction predicted by Hall and Padgett between the branch site of the pre-mRNA substrate and U12 RNA.

Native gel electrophoresis reveals that U11, U12, and U5 snRNPs assemble onto the P120 pre-mRNA to form splicing complexes.

Thus, it is more correct to speak about the splicing machinery which is used to process them, differentiating between U2-type (canonical or major) and U12-type (non-canonical or minor).

In addition, functionally important sequence elements contained within U12-type introns and snRNAs are highly conserved during evolution.

Illustration of exons and introns in pre-mRNA. The mature mRNA is formed by splicing.
A comparison between major and minor splicing mechanisms
U1 and U11 can be folded similarly