Enzymatically active intermediate in the conversion between the low and high molecular weight DNA polymerase.

Abstract

MOST eukaryotic cells contain at least two types of DNA polymerase—a low molecular weight (3.5S) enzyme found predominantly in the nucleus and a heterogenous high molecular weight (6–8S) enzyme(s) isolated from the cytoplasmic fraction1–4. Although the physiological relationship between these activities is not yet understood, recent studies have shown that it is the cytoplasmic polymerase that responds to the stimulus for cellular proliferation5,6. The role this enzyme could play in DNA replication in vivo is not certain; most DNA is located in the nucleus and the nuclear (3.5S) enzyme can catalyse a single round of complementary synthesis in vitro1. One way to resolve this apparent paradox is to propose a precursor relationship between the cytoplasmic and nuclear DNA polymerases. Such a model is supported by evidence for the existence of a common component in the two enzymes. Chang and Bollum8, using rabbit antibody prepared against a high molecular weight DNA polymerase, have found common antigenic determinants existing in both activities. We have shown that an active 3.5S component can be dissociated from the high molecular weight activity by the addition of salt. This subunit appears identical to the nuclear 3.5S enzyme by several enzymatic and physical criteria9,10. Furthermore, this subunit can be reconverted reversibly to a high molecular weight DNA polymerase by the removal of the salt4,9. I present here evidence for the existence of an intermediate in the in vitro conversion between the low and high molecular weight DNA polymerases.