Homology in accessory proteins of replicative polymerases—E.colito humans

The basis for the remarkably high processivity of DNA polymerases that duplicate long chromosomes appears quite similar in prokaryotes and eukaryotes. In each of these cell types, the replicative polymerase has several accessory proteins which endow the polymerase subunit with its speed and processivity. The replicative polymerases of the well studied systems of bacteriophage T4, E.coli (DNA polymerase HI holoenzyme) and humans (polymerase 6), contain accessory proteins which form a 'sliding clamp' on DNA that acts to tether the polymerase to the DNA for rapid and highly processive synthesis (1-4). In the E.coli system this sliding clamp has been shown to be a dimer of the /? subunit, which is in the shape of a ring encircling the DNA (5). The functional homologue of /3 in the T4 system is the product of gene 45 (g45 protein) and in humans it is the proliferating cell nuclear antigen (PCNA). These proteins are homologous in function, and although they show no homology at the amino acid sequence level, a case has recendy been made for a structural similarity of /S to PCNA and to the T4 g45 protein on the basis of sequence using the crystal structure of /3 as a guide (5).