We studied the mutation effect of one of the putative loop residues Thr792 in human DNA topoisomerase IIalpha (TOP2alpha). Thr792 mutants were expressed from high or low copy plasmids in a temperature sensitive yeast strain deficient in TOP2 (top2-1). When expressed from a high copy plasmid, mutants with small side chains complemented the yeast defect; however, from a low copy plasmid, only wild-type, Ser, and Cys substitution mutants complemented the yeast defect. Interestingly, at-the permissive temperature other mutants (e.g., Val, Gly, and Glu substitutions) showed the dominant negative effect to the top2-1 allele, which was not observed by the control alpha4-helix mutants. T792E mutant was 10-fold less active than wild-type and the T792P had no decatenation activity in vitro. These results suggest that Thr792 in human TOP2a is involved in enzyme catalysis. (C) 2003 Elsevier Science (USA). All rights reserved.