Which small nuclear RNA is involved in the assembly of spliceosomes?

The small nuclear RNA that is primarily involved in the assembly of spliceosomes is U1. U1 snRNA plays a critical role in the spliceosome's function by base pairing with the 5' splice site of pre-mRNA, which marks the initial step in the splicing process. The binding of U1 to the mRNA is crucial for the identification of splice sites, and it also helps in the recruitment of other spliceosomal components necessary for splicing.

In the formation of spliceosomes, U1 snRNA is recognized as one of the first RNA components to associate with the pre-mRNA transcript, establishing the scaffold necessary for the further assembly of additional snRNPs (small nuclear ribonucleoproteins) and other proteins that facilitate the splicing reaction. U2, U4, and U6 also play essential roles in subsequent steps of spliceosome assembly and function, such as the recognition of the branch point and catalysis of the splicing reaction, but U1 is pivotal in the initial recognition and binding process at the splice junctions. This makes U1 the foundational snRNA in the assembly of the spliceosome.