Identification and amplification of the E. coli phr gene product

We have constructed a series of multicopy plasmids that complement mutations in the phr gene of Escherichia coli. By subcloning into a tac plasmid vector we obtained a phr plasmid that upon induction overproduces two proteins of Mr's 49,000 and 20,000. Tn1000 insertions into the phr gene caused the disappearance of the 49,000 dalton protein, thus demonstrating this protein to be the phr gene product, DNA photolyase. The photolyase encoded by the phr gene makes up about 15% of total cellular proteins after induction of cells carrying a tac-phr plasmid. This protein binds specifically to UV (254 nm) irradiated DNA and upon exposure to near UV (300-500 nm) illumination repairs the UV damage and dissociates from DNA.