Preface

Splicing is a key step in the gene expression of eukaryotic cells which needs an exact definition by splicing signals. By requiring accurate exon-intron boundary definitions the need for controling is essential for the gene expression. Current analyses show that this control is done by weak consensus signals which are widespread across eukaryotic groups, indicating that a common ancestor had similiar splicing signals to those of metazoans, especially in regard to the fact that the branch site is highly conserved in some (but by no means every) fungi.[8]
The strength of the polypyrimidine tract (PPT) - which is important for the recognition of the 3' splice site - differs across eukaryotes, with fungi often possesing weaker PPTs than those of metazoans. Furthermore, the strength of a PPT correlates with the changes in a U2 auxiliary factor (U2AF)[10] which indicates that their evolutionary pathway is related.
The arginine-serine-rich (RS) domain at the N terminus of U2AF65 contacts the branch site during sliceosome assembly and is neccessary for the proper pairing of U2 snRNA branch site base pairing.
Newer experiments show that serine-arginine-rich (SR) proteins provide a similiar mechanism. SR proteins bind to exonic splicing enhancers (ESEs) to recruit parts of the spliceosome through their RS domain.[1]
Another fact is that the ESE-bound SR protein can stabilize the U2 snRNA branch site base pairing during assembly by contacting the branch site and thereby promoting the recognition of suboptimal branch sites.
This processes can be visualized the following way: