: Heavy metals such as Cr2+, Ni2+ Pb2+ and Cd2+ are environmental toxicants, and are regarded as carcinogens. Accumulated evidences have suggested that these metals, at low concentrations, stimulate DNA replication of cultured cells, while they simultaneously inhibit repair processes of DNA damage caused by another agent. Consequently, mutations of various genes such as oncogenes and suppressor genes could be formed, thereby causing cancer. However, the molecular mechanism of mutagenesis by heavy metals remains poorly understood. Such mutations are most likely to be resulted from modification of the replication machinery to facilitate error-prone DNA synthesis bypassing the damaged bases by an unidentified protein(s) that, in the presence of heavy metals, (a) binds to damaged DNA, (b) unwinds damaged DNA and/or (c) stimulates error-prone DNA polymerases for translesion DNA synthesis. The PT's laboratory has recently discovered that purified lipocortin I heterotetramer, a nuclear protein which is capable of binding various heavy metals, shares some common features with replication protein A (RP-A), a single strand DNA binding protein (SSB) essential for multiple aspects of DNA metabolism including repair, recombination and replication. We hypothesized that lipocortin I (and related II) heterotetramer alters the replicative machinery by replacing RP-A under the influence of heavy metals. Employing purified lipocortin heterotetramers and error-prone DNA polymerase (Pol) a, B or TI with chemically modified polynucleotides as damaged DNA templates, we will perform the following specific aims: (1) to investigate binding of lipocortins to damaged DNA in the presence of heavy metals, (2) to examine unwinding of damaged DNA by lipocortins in the presence of heavy metals, and (3) to study stimulation of Pol a, B and n by lipocortins for translesion DNA synthesis in the presence of heavy metals. Our proposed research will provide further insights to understanding of the molecular mechanisms of carcinogenesis, genetic instability and pathological diseases associated with the formation of mutations by heavy metals.