The Chinese hamster ovary (CHO) cell mutant EM9 is hypersensitive to ethyl methanesulfonate (EMS) (10-fold) and ionizing radiation (1.8-fold), and it is unable to grow in medium containing chlorodeoxyuridine (CldUrd) under conditions in which 20% of genomic Thy is replaced by chlorouracil during DNA replication (9, 29). EM9 repairs -y-ray and EMS-induced single-strand breaks at a reduced rate and exhibits a 10-fold increase in the occurrence of sister chromatid exchange (SCE) (27,28). The sensitivity of EM9 to alkylating agents is suggestive of a defect in the base excision repair pathway, which involves sequential action by DNA glycosylase, apurinic-apyrimidinic endonuclease, deoxyribose-phosphodiesterase, DNA polymerase, and DNA ligase activities (17). The reduced rate of single-strand break rejoining suggests that the defect in EM9 lies within a postincision step of this pathway. EM9 is phenotypically similar to cells derived from individuals with Bloom's syndrome (BS), a cancer-prone autosomal recessive disorder characterized by high SCEs (10-fold) and sensitivity to alkylating agents (4,14,15 (31). As yet, no specific role has been identified for DNA ligase III (31). Altered DNA ligase activity in BS cells has been observed in several studies, in which it was proposed that the activity of DNA ligase I was affected (5, 6, 33, 34), but no major abnormality in DNA ligase activity was found in the one reported study in which EM9 was examined (7). However,

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A methyl methane sulfonate (MMS)-sensitive mutant of Escherichia coli AB 1157 was obtained by N-methyl-N'-nitro-N-nitrosoguanidine treatment. The mutant strain, AB 3027, is defective both in endonuclease activity for apurinic sites in deoxyribonucleic acid (DNA) and in DNA polymerase I, as shown by direct enzyme assays. Derivative strains, which retained the deficiency in endonuclease activity for apurinic sites (approximately 10% of the wild-type enzyme level) but had normal DNA polymerase I activity, were obtained by P1mediated transduction (strain NH5016) or by selection of revertants to decreased MMS sensitivity. These endonuclease-deficient strains are more MMS-sensitive than wild-type strains. Revertants of these deficient strains to normal MMS resistance were isolated. They had increased levels of the endonuclease activity but did not attain wild-type levels. The data suggest that endonuclease for apurinic sites is active in repair of lesions introduced into DNA as a consequence of MMS treatment. Two different endonucleases that specifically attack DNA containing apurinic sites are present in E. coli K-12. A heat-labile activity, sensitive to inhibition by ethylenediaminetetraacetate, accounts for 90% of the total endonuclease activity for apurinic sites in crude cell extracts. The residual 10% is due to a more heat-resistant activity, refractory to ethylenediaminetetraacetate inhibition. The AB3027 and NH5016 strains have normal amounts of the latter endonuclease but no or very little of the former activity. 646 on August 1, 2020 by guest

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