Alternative RNA Splicing

RNA splicing is a post-transcriptional mechanism where precursor mRNA, or pre-mRNA, is converted into mature mRNA by removing the introns and re-joining the exons. 

Constitutive RNA splicing joins each exon in their order in the gene to produce a single type of mature mRNA. For example, in a eukaryotic gene with five exons, numbered 1 to 5, constitutive splicing of pre-mRNA will result in a mature mRNA with five exons.

In contrast, alternative RNA splicing uses a single gene to produce multiple types of proteins by joining or removing different combinations of exons and introns to produce distinct mature mRNAs. 

A gene containing five exons might be spliced into a mature mRNA with exons 1 and 5; exons 2, 3 and 5; or exons 1, 3 and 4.  In some cases, the mature mRNA may even contain a retained intron. These variations allow eukaryotic cells to produce a larger assortment of proteins than the number of genes present in their DNA.

Alternative mRNA splicing allows the same gene to produce different tissue-specific forms of the mature mRNA. For example, different variants of alpha tropomyosin are expressed in smooth muscle cells, striated muscle cells, and brain cells.

Alternative RNA splicing is tightly controlled and is regulated by a set of proteins known as activators and repressors. 

Activators bind to specific sequences on the pre-mRNA exons and introns called exonic or intronic splicing enhancers. This binding allows the spliceosome to recognize weak splice sites that are not recognized in constitutive splicing. 

In contrast, repressors bind to exonic and intronic splicing silencers.  This prevents the spliceosome from assembling, causing it to skip over specific splice sites.