The gene TOP2 encoding yeast topoisomerase II has been cloned by immunological 
screening of a yeast genomic library constructed in the phage lambda expression 
vector, lambda gt11. The ends of the message encoded by the cloned DNA fragment 
were delimited by the Berk and Sharp procedure (S1 nuclease mapping) for the 5' 
end and mapping of the polyA tail portion of a cDNA fragment for the 3' end. The 
predicted size of the message agrees with the length of the message as 
determined by Northern blot hybridization analysis. The identity of the gene was 
confirmed by expressing the gene in E. coli from the E. coli promoter lac UV5 to 
give catalytically active yeast DNA topoisomerase II. Disruption of one copy of 
the gene in a diploid yeast creates a recessive lethal mutation, indicating that 
the single DNA topoisomerase II gene of yeast has an essential function.