TRANSCRIPTION-COUPLED REPAIR OF PSORALEN CROSS-LINKS BUT NOT MONOADDUCTS IN CHINESE-HAMSTER OVARY CELLS

We have examined the rate and extent of removal of 4'-(hydroxymethyl)- 4,5',8-trimethylpsoralen (HMT) cross-linkable monoadducts and interstr and cross-links from restriction fragments within the amplicon contain ing the dihydrofolate reductase (DHFR) gene in the Chinese hamster ova ry (CHO) cell line B-11. The rate and extent of removal of HMT cross-l inks was significantly greater in an actively transcribed fragment tha n in a nontranscribed extragenic fragment of similar size. For the 5' half of the DHFR gene, approximately 80% of the HMT cross-links were r emoved in 8 h, in agreement with results reported by Vos and Wauthier [Vos, J. M., and Wauthier, E. L. (1991) Mol. Cell Biol. 11, 2245-2252, 1991]. However, few cross-links were removed in that period from the nontranscribed fragment, whose 5' end is approximately 7 kb downstream from the DHFR transcription unit and which includes a putative replic ation initiation Site. Even after 24 h, only about 50% of the cross-li nks had been removed from this fragment. In contrast, both the rate an d the extent of removal of cross-linkable HMT monoadducts were similar in the two fragments with 50% of the cross-linkable monoadducts remov ed in 24 h. Moreover, monoadducts formed in the bulk of the genome wer e removed in 24 h. Moreover, monoadducts formed in the bulk of the gen ome were removed at a slightly slower rate and to a lesser extent (30% in 24 hours) than those from either of these specific sequences. Thes e results suggest that HMT cross-links are more efficiently removed fr om actively transcribed sequences than from nontranscribed sequences, but that the transcriptional state does not strongly influence the rem oval of cross-linkable monoadducts.