Camptothecin-induced in vivo topoisomerase I cleavages in the transcriptionally active tyrosine aminotransferase gene.

The topoisomerase I inhibitor camptothecin induces the formation of covalent topoisomerase I-DNA intermediates in vitro. In vivo these intermediates are produced upon administration of camptothecin to FTO-2B cells, as demonstrated by the occurrence of single-stranded DNA breakages that have protein covalently associated with the free 3' end and by the covalent association of approximately 50% of nuclear topoisomerase I with DNA. We have analyzed the frequency and distribution of camptothecin-induced topoisomerase I strand cleavages in the transcriptionally active rat tyrosine aminotransferase gene. Cleavages are largely confined to the transcribed region and occur predominantly on the lower strand. Increasing the transcriptional activity of the gene with either glucocorticoids or cAMP increases the intensity but does not change the position of camptothecin-induced cleavages. Camptothecin treatment decreases tyrosine aminotransferase mRNA levels and inhibits transcription. These observations suggest that topoisomerase I participates in transcriptional elongation.