A mutant murine cell line has previously been reported to be resistant to the 
AT-specific DNA minor groove ligand 2',5'-bi-1H-benzimidazole, 
2',(4-ethoxyphenyl)-5-(4-methyl-1-piperazinyl), trichloride (Ho33342), due to an 
enhanced capacity to remove ligand molecules from cellular DNA via a pathway 
which can be blocked by DNA topoisomerase poisons. We have studied the 
relationship between ligand resistance and DNA topoisomerase II activity. The 
cross-sensitivity patterns of the mutant were examined for covalently 
(anthramycin) and non-covalently (distamycin A) binding minor groove ligands, 
and DNA intercalating [adriamycin, mitoxantrone and 
4'-(9-acridinylamino)methanesulphon-m-anisidide (mAMSA)] and non-intercalating 
(VP16-213) topoisomerase II poisons. The mutant was cross-resistant to 
distamycin A alone. The mutant showed no abnormality in: (i) the in vitro 
decatenation activity of topoisomerase II, (ii) VP16-213 or mAMSA induced 
protein-DNA cross-linking activities in nuclear extracts, (iii) 'cleavable 
complex' generation (or DNA strand scisson) in intact cells exposed to 
topoisomerase poisons. Ho33342 and the topoisomerase II inhibitor novobiocin 
were found to disrupt both the in vitro binding of nuclear extracted proteins, 
from mutant and parental cells, to plasmid DNA and the formation of drug-induced 
cleavable complexes in vitro. Unexpectedly, Ho33342 induced significant levels 
of DNA-protein crosslinking in both parental and mutant cells. We conclude that: 
(i) resistance of the mutant is limited to non-covalently binding minor groove 
ligands, (ii) Ho33342 can block the trapping of DNA topoisomerase II by enzyme 
poisons in vitro, (iii) Ho33342 can induce a novel form of DNA-protein 
cross-link in intact cells, and (iv) the resistance of the mutant is not 
dependent upon some abnormality in topoisomerase II function.