The carboxyl terminus of the bacteriophage T4 DNA polymerase contacts its sliding clamp at the subunit interface

The location of the interaction of the COOH terminus of the bacteriophage T 4 DNA polymerase with its trimeric, circular sliding clamp has been establi shed. A peptide corresponding to the COOH terminus of the DNA polymerase wa s labeled with a fluorophore and fluorescence spectroscopy used to show tha t it forms a specific complex with the sliding clamp by virtue of its low K -D value (7.1 +/- 1.0 mu M). The same peptide was labeled with a photoaffin ity probe and cross-linked to the sliding clamp. Mass spectrometry of trypt ic digests determined the sole linkage point to be Ala-159 on the sliding c lamp, an amino acid that lies on the subunit interface. These results demon strate that the COOH terminus of the DNA polymerase is inserted into the su bunit interface of its sliding clamp, thereby conferring processivity to th e DNA polymerase.