During the C complex formation, which snRNP replaces U1?

In the context of mRNA splicing, the formation of the C complex is a critical step involving small nuclear ribonucleoproteins (snRNPs). During this process, the spliceosome undergoes several changes and rearrangements as it assembles on the pre-mRNA.

The correct replacement involves U1 snRNP being initially bound to the 5' splice site. When the spliceosome progresses to the C complex stage, U1 is replaced by the U6 snRNP. This transition is essential as U6 plays a crucial role in the catalytic activity of the spliceosome, particularly in facilitating the formation of the lariat structure and ensuring the proper alignment of the splice sites for effective splicing to occur.

The presence of U6 is fundamental to the mechanics of splicing. It helps in stabilizing the spliceosomal structure and provides essential RNA-RNA interactions that enable the correct positioning of the reactants necessary for the splicing reaction. U6, along with U2, forms critical interactions that enhance the spliceosomal assembly and promote the splicing process.

While U2, U4, and U5 snRNPs also have important roles in the spliceosome, they do not directly replace U1 in this specific step