RNA Processing I: Splicing (CMB part 12)

Nuclear mRNA precursors undergo splicing through a lariat-shaped or branched intermediate. In addition to the consensus sequences at the 5′ and 3′ ends of nuclear introns, branchpoint consensus sequences are also present. In yeast, this sequence is almost invariant as UACUAAC, whereas in higher eukaryotes, the consensus sequence is more variable, represented as YNCURAC. In all cases, the branched nucleotide corresponds to the final A in the sequence. The yeast branchpoint sequence also determines which downstream AG serves as the 3′ splice site.Splicing occurs on a complex structure known as the spliceosome. Yeast and mammalian spliceosomes have sedimentation coefficients of approximately 40S and 60S, respectively. Genetic studies have revealed that base pairing between U1 snRNA and the 5′ splice site an mRNA precursor is necessary but not sufficient for splicing. The U6 snRNP also forms a base-pairing association with the 5′ end of the intron, which begins before the formation of the lariat intermediate but may alter its nature after this initial step. This interaction between U6 and the splicing substrate is critical for the splicing process. Furthermore, U6 interacts with U2 during splicing.The U2 snRNA base-pairs with the conserved sequence at the splicing branchpoint, an interaction essential for splicing. Additionally, U2 forms significant base pairs with U6 to create a region referred to as helix I, which plays a role in aligning these snRNPs for the splicing process. The U4 snRNA base-pairs with U6, contributing to the splicing mechanism.