The spliceosome is the catalytic entity that removes the introns from the primary transcripts in eukaryotes. The spliceosome consists of four small nuclear ribonucleoproteins (snRNPs), named U1,U2, U4/U6, and U5 snRNP, and numerous non-snRNP proteins. Each snRNP consists of one (U1, U2, and U5 snRNP) or two (U4/U6 snRNP) RNA molecules and a large number of different proteins. Within the U4/U6 snRNP, U4 and U6 snRNA are basepaired together. Under conditions of in vitro splicing reactions in HeLa cell nuclear extracts (i.e., at about 150 mMsalt concentration), the spliceosomal snRNPs are organized in three RNP forms: 12S U1, 17S U2, and 25S [U4/U6.U5] trisnRNP. These snRNP complexes may be considered as functional subunits of the spliceosome.
