Editing Alternative splicing (section)

====Exon skipping: ''Drosophila'' ''dsx''====
[[File:Dsx splicing.jpg|thumb|Alternative splicing of ''dsx'' pre-mRNA]]
Pre-mRNAs from the ''D. melanogaster'' gene ''[[doublesex|dsx]]'' contain 6 exons. In males, exons 1,2,3,5,and 6 are joined to form the mRNA, which encodes a transcriptional regulatory protein required for male development. In females, exons 1,2,3, and 4 are joined, and a [[polyadenylation]] signal in exon 4 causes cleavage of the mRNA at that point. The resulting mRNA is a transcriptional regulatory protein required for female development.<ref name=Lynch>{{cite journal | vauthors = Lynch KW, Maniatis T | title = Assembly of specific SR protein complexes on distinct regulatory elements of the Drosophila doublesex splicing enhancer | journal = Genes & Development | volume = 10 | issue = 16 | pages = 2089–101 | date = August 1996 | pmid = 8769651 | doi = 10.1101/gad.10.16.2089 | doi-access = free }}</ref>

This is an example of exon skipping. The intron upstream from exon 4 has a [[polypyrimidine tract]] that doesn't match the [[consensus sequence]] well, so that U2AF proteins bind poorly to it without assistance from splicing activators. This 3' splice acceptor site is therefore not used in males. Females, however, produce the splicing activator Transformer (Tra) (see below). The SR protein Tra2 is produced in both sexes and binds to an ESE in exon 4; if Tra is present, it binds to Tra2 and, along with another SR protein, forms a complex that assists U2AF proteins in binding to the weak polypyrimidine tract. U2 is recruited to the associated branchpoint, and this leads to inclusion of exon 4 in the mRNA.<ref name=Lynch/><ref name=Graveley>{{cite journal | vauthors = Graveley BR, Hertel KJ, Maniatis T | title = The role of U2AF35 and U2AF65 in enhancer-dependent splicing | journal = RNA | volume = 7 | issue = 6 | pages = 806–18 | date = June 2001 | pmid = 11421359 | pmc = 1370132 | doi = 10.1017/S1355838201010317 }}</ref>