title: Replication intermediates formed during initiation of DNA synthesis in methotrexate-resistant CHOC 400 cells are enriched for sequences derived from a specific, amplified restriction fragment.
abstract: 1-beta-D-Arabinofuranosylcytosine (ara-C) inhibits nuclear DNA replication in Chinese hamster ovary cells by an efficient chain termination mechanism without affecting the rate at which cells traverse G1 and enter S [Heintz, N. H., & Hamlin, J. L. (1983) Biochemistry 22, 3557-3562]. Here we have employed ara-C to enrich for replication intermediates formed during initiation of DNA synthesis in synchronized CHOC 400 cells, a methotrexate-resistant derivative of Chinese hamster ovary cells that contains approximately 1000 copies of an early replicating 150-kb chromosomal domain. This highly amplified domain includes the gene for dihydrofolate reductase (DHFR). CHOC 400 cells were collected at the G1/S boundary of the cell cycle with aphidicolin prior to release into S in the presence of both [methyl-3H] thymidine and various concentrations of ara-C. Chromatographic fractionation of restriction endonuclease digests over benzoylated naphthoylated DEAE-cellulose (BND-cellulose) showed that high concentrations of ara-C inhibited the maturation of chromosomal replication intermediates containing ssDNA (replication forks) into dsDNA for up to 60 min. The effect of ara-C on the sequence complexity of replication intermediates formed during early S phase was determined by hybridizing purified intermediates labeled with 32P in vitro to Southern blots of genomic DNA derived from both methotrexate-sensitive and methotrexate-resistant Chinese hamster ovary cells. In the absence of ara-C, 32P-labeled ssDNA BND-cellulose fractions from cultures released into S for 30-60 min hybridized to a spectrum of restriction fragments encompassing 40-50 kb of the amplified DHFR domain.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000818:Animals; D001483:Base Sequence; D002453:Cell Cycle / Q000187:drug effects; D002460:Cell Line; D006224:Cricetinae; D003412:Cricetulus; D003561:Cytarabine / Q000494:pharmacology; D004247:DNA / Q000302:isolation & purification; D004261:DNA Replication; D004262:DNA Restriction Enzymes; D004351:Drug Resistance; D005260:Female; D005784:Gene Amplification; D005796:Genes / Q000187:drug effects*; D006918:Hydroxyurea / Q000494:pharmacology; D007700:Kinetics; D008727:Methotrexate / Q000494:pharmacology*; D010053:Ovary; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics*
chemical_list: D003561:Cytarabine; D004247:DNA; D013762:Tetrahydrofolate Dehydrogenase; D004262:DNA Restriction Enzymes; D006918:Hydroxyurea; D008727:Methotrexate