Studies with yeast DNA topoisomerase mutants indicate that neither topoisomerase 
I nor II appears to be essential for transcription by RNA polymerase II. 
However, plasmids carrying transcriptionally active genes are found to be 
extremely negatively supercoiled when isolated from mutants lacking 
topoisomerase I. Supercoiling occurs during transcriptional elongation rather 
than during transcriptional activation. It takes place in the absence of 
topoisomerase I and does not seem to be dependent on topoisomerase II since it 
can occur at the nonpermissive temperature in a top1-top2 ts mutant. Whether 
this change in linking number is due to an unusual form of topoisomerase II or 
whether it is due to a new enzyme has yet to be determined. The results suggest 
that topoisomerase I is normally required to relax transcriptionally induced 
supercoils. A model is discussed which considers the role of topoisomerases in 
the movement of RNA polymerase along the DNA template.