We have isolated mutants defective in DNA topoisomerases and an endonuclease 
from the fission yeast Schizosaccharomyces pombe by screening individual 
extracts of mutagenized cells. Two type I topoisomerase mutants (top1) and three 
endonuclease mutants (end1) were all viable. The double mutant top1 end1 was 
also viable and, in its extract, Mg2+- and ATP- dependent type II activity could 
be detected. Three temperature-sensitive (ts-) mutants having heat-sensitive 
(hs-) type II enzymes were isolated, and the ts- marker cosegregated with the 
hs- type II activity. All the ts- mutations fell in one gene (top2) tightly 
linked to leul in chromosome II. The nuclear division of single top2 mutants was 
blocked at the restrictive temperature, but the formation of a septum was not 
inhibited so that the nucleus was cut across with the cell plate. In contrast, 
the double top1 top2 mutants were rapidly arrested at various stages of the cell 
cycle, showing a strikingly altered nuclear chromatin region. The type II 
topoisomerase may have an essential role in the compaction and/or segregation of 
chromosomes during the nuclear division but also complement the defect of the 
type I enzyme whose major function is the maintenance of chromatin organization 
throughout the cell cycle.