By-pass of the major aminofluorene-DNA adduct during in vivo replication of single- and double-stranded øX174 DNA treated with N-hydroxy-2-aminofluorene

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Abstract
To examine the effects of aminofluorene-DNA adduct formation on the biological activity of DNA, single-stranded (ss) øX174 DNA and øX174 replicative form (RF) DNA were modified to different extents with 3 H-labeled N-hydroxy-2-aminofluorene and subsequently transfected to Escherichia coli spheroplasts with different repair capabilities. When the fraction of active ss øX174 DNA molecules was measured as a function of the mean number of adducts per molecule, exponential survival curves were obtained from which it could be deduced that in wild-type, uvrA− and recA− cells at least 86%, and in uvrC− cells at least 82% of the introduced adducts do not cause inactivation. In the case of RF DNA the survival curves are non-exponential, but they nevertheless show that an exceptionally high number of adducts per RF molecule must be introduced to destroy its biological activity. On average 52 adducts per RF molecule were needed to reduce the survival to 37%, irrespective of whether wild-type, uvrA− or recA− cells were used. On the other hand, the survival of the uvrC− cells was considerably lower, but even in these cells a majority of the adducts is not lethal. By h.p.l.c. analysis of the modified DNA after hydrolysis with trifluoro-acetic acid, 81 and 84% of the adducts in ss- and RF DNA, respectively, could be identified as N-(guanin-8-yl)-2-aminofluorene. The results strongly indicate that this type of major modification product is very frequently by-passed during replication of both single- and double-stranded DNA. The results together with the data obtained by sucrose gradient analysis both before and after an alkali treatment and those obtained by h.p.l.c. analysis suggest that inactivation of ssDNA is mainly due to minor modifications such as secondary lesions consisting of chain breaks and alkali-labile sites together with unidentified interaction products.

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