In article <Pine.HPP.3.91.970218134513.23388B-100000 at fhs.csu.McMaster.CA>,
Martin Hughes <mhughes at fhs.csu.McMaster.CA> wrote:
>On Mon, 17 Feb 1997, Dr. Duncan Clark wrote:
>>> Results I have seen published with mixes using Taq and a proof-reading
>> enzyme all give around the same increase in fidelity ie approx 3 fold.
>> Figures for Pfu on its own versus Taq vary from 10 to 12 fold
>>I'm interested to know why this should be the case. As I
>understand it, the proofreading enzymes have their exonuclease activity
>as a subunit of the enzyme, which 'proofreads' as the strand is
>synthesised, comparing the daughter strand to the parent. If this is the
>case (please correct me if I'm wrong), how
>(mechanistically) can a 1/50th or so addition of the proofreading enzyme
>do anything to the strands which are being synthesised by the Taq? Does
>it 'scan' already synthesised double-stranded molecules, looking for an
>error during extension?
>>Thanks in advance for any light you can shed on this
>>Dr. Martin Hughes
>Department of Biochemistry
>Hamilton, ONTARIO, Canada.
Taq has VERY low processivity (that's probably spelt wrong, but) and thus
adds only a few bases at a time before hopping off the DNA strand. Another
molecule of Taq comes along and continues. If the Taq molecule dissociates
after making a mismatch mistake, it is more likely that a polymerase with
an exo activity will bind, allowing the mismatch to be taken care of
before DNA synthesis continues. The view of a primer binding to a single
stranded DNA molecule and a SINGLE Taq molecule extending that primer to
the end of the ss DNA is not how it actually works mechanistically.