R.CviJI is unique among site-specific restriction endonucleases in that its activity can be modulated to recognize either a two or three base sequence. Normally R.CviJI cleaves RGCY sites between the G and C to leave blunt ends. In the presence of ATP R.CviJI(*) cleaves RGCN and YGCY sites, but not YGCR sites. The gene encoding R.CviJI was cloned from the eukaryotic Chlorella virus IL-3A and expressed in Escherichia coli. The primary E.coli cviJIR gene product-is a 278 amino acid protein initiated from a GTG codon, rather than the expected 358 amino acid protein:initiated from an in-frame upstream ATG codon. Interestingly, the 278 amino acid protein displays the normal restriction activity but not the R.CviJI(*) activity of the native enzyme. Nine restriction and modification proteins which recognize a central GC or CG sequence share short regions of identity with R.CviJI amino acids 144-235, suggesting that this region is the recognition and/or catalytic domain.
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