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PstI is a type II restriction endonuclease isolated from the Gram negative species, Providencia stuartii. It is the isoschizomer of Streptomyces albus P restriction enzyme SalPI. It cleaves DNA at the recognition sequence 5′-CTGCA/G-3′ generating fragments with 3′-cohesive termini. This cleavage yields sticky ends 4 base pairs long. PstI is catalytically active as a dimer. The two subunits are related by a 2-fold symmetry axis which in the complex with the substrate coincides with the dyad axis of the recognition sequence. It has a molecular weight of 69,500 and contains 54 positive and 41 negatively charged residues.
The PstI restriction/modification (R/M) system has two components: a restriction enzyme that cleaves foreign DNA, and a methyltransferase which protect native DNA strands via histone methylation. The combination of both provide a defense mechanism against invading viruses. The methyltransferase and endonuclease are encoded as two separate proteins and act independently. In the PstI system, the genes are encoded on opposite strands and hence must be transcribed divergently from separate promoters. The transcription initiation sites are separated by only 70 base pairs. A delay in the expression of the endonuclease relative to methylase is due to the inherent differences of the two proteins. The endonuclease is a dimer, requiring a second step for assembly, whereas the methylase is a monomer.
PstI is functionally equivalent to BsuBI. Both enzymes recognize the target sequence 5'CTGCAG. The enzyme systems have similar methyltransferases (41% amino acid identity), restriction endonucleases (46% amino acid identity), and genetic makeup (58% nucleotide identity). These observations suggest a shared evolutionary history.
When examining the preferential double strand cleavage of DNA, the restriction endonuclease PstI bind to pSM1 plasmid DNA.

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  1. Joe Link

    Update  Scheduled Maintenance - Thursday, June 27th, 2019 @ 9PM PST

    Northwest Firearms will be down for scheduled maintenance Thursday, June 27th, 2019 beginning at 9PM PST. A two-hour window is allocated, though the actual downtime should be much less :)
  2. Joe Link

    Announcement  Northwest Firearms V4 Upgrade - Monday, July 15th @ 3AM PST

    I'm happy to announce the tentative date and time of the Northwest Firearms V4 Upgrade is Monday, July 15th, beginning at 3AM PST. While we hope get the site back up and running as soon as possible, it's hard to predict exactly how long the site will be down. If everything goes as planned it...
  3. Weebs

    Which scope for which rifle (rpr/ar10 - pst ii/xtr)

    I’m looking for help from the brain trust here. I just picked up a vortex viper pst ii and I’m trying to decide which rifle to put it on. Both are good candidates. My goal from both rifles is to be proficient at 600 yards from a solid rest with a dream of doing some 1000 yard shots. (Though...
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