Topoisomerase II question

Scott Smith x91smith41 at
Sun Feb 18 20:13:07 EST 1996

Hi y'all

I'm presenting a paper for my Human Genetics class this week on
the effects of UV radiation on the activity of D. melanogaster
topoisomerase II.  The paper (by Anita H. Corbett, et al) has me
confused on a couple of points.

First of all, it's my understanding that topoisomerase II adds
negative superhelical turns to DNA molecules.  However, the paper
keeps talking about it's ability to "relax negatively supercoiled DNA".
Most of my CMB and molecular genetics texts indicate that gyrase
operates by _adding_ negative supercoils.  My Human Genetics text, though,
says that gyrase "unwinds the supercoiled turns of the right-handed
double helix, creating a "relaxed" but left-handed or negative,
supercoiled double helix."  Maybe there is just some confusion about
the terminology being used.

The authors of the paper break the activity of topoisomerase II into
6 steps, including DNA binding, prestrand passage cleavage/religation,
strand passage, poststrand passage cleavage/religation, ATP hydrolysis,
and recycling of the enzyme.

I'm stumped about why there are two cleavage/religation events, one
on either side of the actual passage.  The article doesn't go into
any more detail, and I can't track down the reference they use for
this information.  Unless this is another instance in genetics where
probability plays a big role (ie, "keep nicking and resealing with
enough enzymes on enough DNA molecules, and a strand will pass through
somewhere") I can't figure out what the deal is.

Any help would be greatly appreciated, especially good references!

Email replies to x91smith41 at or sgsmith2 at



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