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Heterogeneous Repair of Methylnitrosourea-induced Alkali-labile Sites in Different DNA Sequences¹

Department of Structural and Cellular Biology, University of South Alabama, Mobile, Alabama 36688 [S. P. L., M. T., G. L. W.], and Laboratory of Molecular Pharmacology, National Cancer Institute, NIH, Bethesda, Maryland 20892 [V. A. B.]

The repair of DNA damage induced by methylnitrosourea (MNU) in restriction fragments containing the dihydrofolate reductase (DHFR) gene in Chinese hamster ovary cells was compared to that in equal size restriction fragments containing a nontranscribed flanking sequence 3' to the DHFR gene or the c-fos gene. Following exposure to 10^-3 M MNU, restriction fragments containing either the DHFR gene or the 3' flanking sequence had similar amounts of alkali labile sites, 2 sites/restriction fragment. Fragments encompassing the c-fos gene had less than 2 breaks/fragment. Twenty-four h after exposure to MNU a consistent, but slight and not statistically significant, difference was seen with more adducts removed from the DHFR gene than the 3' flanking sequence. No repair was detected in the c-fos containing fragments. In addition, the repair of N⁷-methylguanine in the overall genome was assessed by use of a ³²P end-labeling technique. Seventy % of this major alkylation product was repaired after 24 h. These findings establish that repair heterogeneity occurs in Chinese hamster ovary cells after exposure to MNU.

¹ This study was supported in part by NIH Grant ES03456.

² Recipient of NIH Postdoctoral Fellowship ES05446.

³ Recipient of NIH Career Development Award ES00150. To whom requests for reprints should be addressed.

Received 4/24/90. Accepted 11/15/90.