What characterizes the early complex in splicing?

The early complex in splicing is characterized by the binding of U1 small nuclear ribonucleoprotein (snRNP) to the 5' splice site of the pre-mRNA. This is a crucial initial step in the assembly of the spliceosome, the complex required for the removal of introns from precursor mRNA.

When U1 binds to the 5' splice site, it helps position the pre-mRNA correctly for the subsequent steps of splicing. This binding is essential for the recognition and interaction of the spliceosomal components with the splice sites, enabling the splicing machinery to accurately identify where introns begin and end.

While U2AF binding to the polypyrimidine tract and other snRNP components like U4 are involved in later stages of spliceosome assembly and function, the defining action of the early complex is the presence of U1 snRNP at the 5' splice site. Therefore, the presence of U1 at this stage is foundational for successful splicing to occur.