Which interaction creates the active site during splicing?

The interaction between U2 and U6 is crucial for the formation of the active site during splicing because they come together to create the catalytic core of the spliceosome. This complex is involved in the precise catalysis of intron removal from pre-mRNA, ensuring that exons are correctly joined together to form mature mRNA.

U2 snRNA binds to the branch point within the intron, and U6 snRNA interacts with U2 and also helps to facilitate the required conformational changes in the spliceosome. Additionally, the U2 and U6 interaction is essential for the positioning of the active site to enable the two transesterification reactions that are necessary for splicing. The first reaction cleaves the 5' exon-intron boundary, while the second reattaches the exons after the intron has been removed.

In contrast, the other options involve interactions that do not directly lead to the formation of the active site necessary for splicing. For example, U1 primarily recognizes and binds the 5' splice site but does not play a direct role in creating the catalytic environment. U4 serves mainly as a regulatory component and is involved in maintaining the stability of U6 until it is needed in the splicing process.