Incision at diaminopurine: thymine base pairs but not at guanine:O4-methylthymine base pairs in DNA by extracts of human cells.

Cell-free extract from the A1235 human malignant glioma cell line was employed to study the possibility of incision at 2,6-diaminopurine:T (DiAP:T), 2-amino-6-methylaminopurine:T (AMAP:T), and G:O4-methylthymine (G:m4T) mismatches, each placed in a 45 bp DNA at a defined site. The incision of a 45 bp DNA containing a G:T mispair at the same site was followed to determine the relationship between base pair structure and repair activity (ies) in the extract. The cell-free extract incised DNAs containing DiAP:T, AMAP:T, and G:T pairs similarly. Reminiscent of the known pattern of incision at G:T mismatches, products from each substrate were consistent with two incisions, one immediately 5' and one immediately 3' to the mismatched T, and only in the strand containing the mismatched T. While DNA with an O6-methylguanine:T (m6G:T) pair was also incised, DNA containing the G:m4T pair was not, but was rendered inciseable by pretreatment with O6-methylguanine-DNA methyltransferase. Incision of DiAP:T-containing DNA by the extract was less in the presence of unlabeled DNA containing G:T mispairs than in the presence of A:T- or G:A-containing DNA or in the absence of competing DNA. We suggest that the mechanism operating on DiAP:T and/or AMAP:T pairs may be the same as the human G:T repair pathway, possibly initiated by the action of a glycosylase as described by Wiebauer and Jiricny [Wiebauer, K., & Jiricny, J. (1989) Nature 339, 234-236; Wiebauer, K., & Jiricny, J. (1990) Proc. Natl. Acad. Sci, U.S.A. 87, 5842-5845].(ABSTRACT TRUNCATED AT 250 WORDS)