Nucleotide excision repair (NER) mechanism is the major pathway responsible for 
the removal of a large variety of bulky lesions from the genome. Two different 
NER subpathways have been identified, i.e. the transcription-coupled and the 
global genome repair pathways. For DNA-damage induced by ultraviolet light both 
transcription-coupled repair and global genome repair are essential to confer 
resistance to cytotoxic effects. To gain further insight into the contribution 
of NER subpathways in the repair of bulky lesions and in their prevention of 
biological effects we measured the rate of repair of dG-C8-AF in active and 
inactive genes in normal human cells, XP-C cells (only transcription-coupled 
repair) and XP-A cells (completely NER-deficient) exposed to NA-AAF. XP-C cells 
are only slightly more sensitive to NA-AAF than normal cells and, like normal 
cells, they are able to recover RNA synthesis repressed by the treatment. In 
contrast, XP-A cells are sensitive to NA-AAF and unable to recover from RNA 
synthesis inhibition. Repair of dG-C8-AF in the active ADA gene proceeds in a 
biphasic way and without strand specificity, with a subclass of lesions quickly 
repaired during the first 8 h after treatment. Repair in the inactive 754 gene 
occurs more slowly than in the ADA gene. In XP-C cells, repair of dG-C8-AF in 
the ADA gene is confined to the transcribed strand and occurs at about half the 
rate of repair seen in normal cells. Repair in the inactive 754 gene in XP-C 
cells is virtually absent. Consistent with these results we found that repair 
replication in XP-C is drastically reduced when compared with normal cells and 
abolished by alpha-amanitin indicating that the repair in XP-C cells is mediated 
by transcription-coupled repair only. Our data suggest that dG-C8-AF is a target 
for transcription-coupled repair and that this repair pathway is the main 
pathway or recovery of RNA synthesis inhibition conferring resistance to 
cytotoxic effects of NA-AAF. In spite of this, repair of dG-C8-AF in active 
genes in normal cells by transcription-coupled repair and global genome repair 
is not additive, but dominated by global genome repair. This indicates that the 
subset of lesions which are capable of stalling RNA polymerase II, and are, 
therefore, a substrate for TCR, are also the lesions which are very efficiently 
recognized by the global genome repair system.