Everything about The Polypyrimidine Tract totally explained
The
polypyrimidine tract is a region of
messenger RNA (mRNA) that promotes the assembly of the
spliceosome, the
protein complex specialized for carrying out
RNA splicing during the process of
post-transcriptional modification. The region is rich with
pyrimidine nucleotides, especially
uracil, and is usually 15-20
base pairs long, located about 5-40 base pairs before the
3' end of the
intron to be spliced.
A number of protein factors bind to or associate with the polypyrimidine tract, including the spliceosome component U2AF and the
polypyrimidine tract-binding protein (PTB), which plays a regulatory role in
alternative splicing. PTB's primary function is in
exon silencing, by which a particular exon region normally spliced into the mature mRNA is instead left out, resulting in the
expression of an
isoform of the protein for which the mRNA
codes. Because PTB is ubiquitously expressed in many higher
eukaryotes, it's thought to suppress the inclusion of "weak" exons with poorly defined
splice sites. However, PTB binding isn't sufficient to suppress "robust" exons.
The suppression or selection of exons is critical to the proper expression of
tissue-specific isoforms. For example,
smooth muscle and
skeletal muscle express alternate isoforms distinguished by mutually exclusive exon selection in alpha-
tropomyosin.
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