Human DNA polymerase was used to copy four stereoisomeric deoxyguanosine (dG) adducts derived from benzo[a]pyrene 7,8-diol 9,10-epoxide (diastereomer with the 7-hydroxyl group and epoxide oxygen trans (BaP DE-2)). The adducts, formed by either cis or trans epoxide ring opening of each enantiomer of BaP DE-2 by N 2 of dG, were placed at the fourth nucleotide from the 5-end in two 16-mer sequence contexts, 5ϳCG*Aϳ and 5ϳGG*T. pol was remarkably error prone at all four diol epoxide adducts, preferring to misincorporate G and A at frequencies 3-to more than 50-fold greater than the frequencies for T or the correct C, although the highest rates were 60-fold below the rate of incorporation of C opposite a non-adducted G. Anti to syn rotation of the adducted base, consistent with previous NMR data for a BaP DE-2 dG adduct placed just beyond a primer terminus, provides a rationale for preferring purine misincorporation. Extension of purine misincorporations occurred preferentially, but extension beyond the adduct site was weak with V max /K m values generally 10-fold less than for misincorporation. Mostly A was incorporated opposite (؉)-BaP DE-2 dG adducts, which correlates with published observations that G 3 T is the most common type of mutation that (؉)-BaP DE-2 induces in mammalian cells.Somatic mutation of proto-oncogenes and tumor suppressor genes is a key component in the initiation of cancer (1). Any DNA damage that escapes repair could lead to mutations. However, replicative DNA polymerases are blocked in a potentially lethal manner when they encounter bulky adducts (2). Thus, they are unable to convert adducts to mutations. Recently, a growing number of DNA polymerases capable of conducting translesion DNA synthesis have been discovered (3-11); these likely hold the key to mutagenesis and the initiation of cancer induced by bulky adducts. One of these lesion-bypassing DNA polymerases, human DNA polymerase eta (pol) 1 (12, 13) is a member of the UmuC/DinB/Rev1/Rad30 superfamily (now called the Y family (14)) of DNA polymerases. In humans it is a product of the XPV skin cancer susceptibility gene; inactivation of pol results in a variant form of xeroderma pigmentosum (12,15). pol incorporates mostly correctly at cis-syn T-T cyclobutane dimers (16,17), N-(deoxyguanosin-8-yl)-acetylaminofluorene (18), and cis-Pt G-G adducts (18,19); small amounts of incorrect nucleotides are also incorporated, mostly at the level seen with undamaged DNA. With (6-4) T-T photoproducts, however, a single G is preferentially incorporated (20). Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental carcinogens (21). Bay-region diol epoxides, formed on an angular benzo-ring, have been shown to be the ultimate carcinogenic metabolites of the PAHs (21-25). Four optically active bay-region diol epoxide isomers (enantiomers of a pair of diastereomers) are formed metabolically from a given hydrocarbon. In the DE-1 ("syn") diastereomer, the epoxide oxygen and benzylic 7-hydroxyl groups are cis, whereas in the DE-2 ("anti") diast...