Disruption of base pairing between the 5' splice site and the 5' end of U1 snRNA is required for spliceosome assembly.

A short RNA oligonucleotide comprising the 5' splice site consensus sequence (5'SS RNA) is sufficient to bind U1 small nuclear ribonucleoprotein particle (snRNP) or to induce the association of U2 snRNP and U4-U5-U6 triple snRNP. Analysis of the requirements of these interactions demonstrates that the 5'SS RNA is recognized independently by at least two different elements during spliceosome assembly: the 5' end of U1 snRNA and a component(s) of the U2-U4-U5-U6 snRNP complex. Since stable 5'SS RNA-U1 snRNA base pairing prevents interaction of the 5'SS RNA with U2-U4-U5-U6 snRNP complex, we conclude that disruption of the initial base pairing between the 5'SS RNA and the 5' end of U1 snRNA is required for subsequent spliceosome assembly.