Repair of DNA adducts of the carcinogen 15,16-dihydro-11-methylcyclopenta[a]phenanthren-17-one in mouse tissues and its relation to tumor induction.

The formation and repair of DNA adducts of the carcinogen, 15,16-dihydro-11-methylcyclopenta[a]phenanthren-17-one, have been studied in three mouse tissues, liver, lung, and skin. Following a single i.m. dose of the carcinogen, DNA adduct formation was observed in all three tissues, the highest being found in liver DNA, the tissue resistant to tumor formation by this carcinogen. In skin and lung, the tissues which were susceptible to tumor formation, binding was approximately one-half that found in liver. Detailed analysis by high-pressure liquid chromatography of the adducts formed in each of the three tissues revealed no major qualitative differences in the eight adduct peaks. In vivo removal of the labeled adducts was studied over a 14-day period following initial treatment, and adducts were analyzed at each time point by high-pressure liquid chromatography. In skin and lung, active removal of the major adducts could not be measured above the normal rate of DNA turnover. By contrast, in the liver, where the rate of DNA turnover was slower, adducts were removed relatively rapidly with a half-life of approximately 2.5 days. Only one minor adduct present in both skin and liver was removed more rapidly than were the major adducts. The results suggest that the persistence of carcinogen-DNA adducts coupled with a relatively high rate of cell division may be related to tissue-specific carcinogenesis by this polycyclic ketone.

Splicing of the late mRNAs of polyoma virus does not occur in the cytoplasm of the infected cell.

The three mRNAs that encode the capsid proteins of polyoma virus are produced by the excision of different sequences from continuous transcripts of the L strand of viral DNA. All three of the mRNAs have long half lives, and the larger species are not converted to the smaller ones to any measurable extent within the cytoplasm. Therefore the cytoplasmic proportions of late polyoma mRNAs are predetermined by splicing that is confined to the nucleus of the infected cell and which is complete by the time that mRNA is transported to the cytoplasm.

Comparison of nuclease digestion of polyoma virus nucleoprotein complex and mouse chromatin.

We digested polyoma virus nucleoprotein complex, isolated from disrupted virions, with micrococcal nuclease and DNase I. The results were compared with digestions of chromatin from mouse nuclei. The nucleosome "core" structures were similar, but the spacing of the nucleosomes in the isolated polymoma nucleoprotein complexes was irregular, whereas in mouse chromatin it was regular. The average nucleosome repeat length in each case was 190 to 200 base pairs. This figure suggests that, unless there are substantial stretches of free DNA, the polyoma nucleoprotein complex contains about 26 nucleosomes. The commonly used method of preparing the nucleoprotein complex by disruption of virions at pH 10.2 may lead to significant damage to the structure. Such damage may be more clearly revealed by the susceptibility of the DNA to nuclease digestion than by the usual criteria of sedimentation velocity and buoyant density.