Why AK is not expressed consistantly?

Marcus, Dr J. J.Marcus at BOTANY.UQ.EDU.AU
Wed Nov 12 23:42:58 EST 1997

If E.coli does not like AK (i.e. it is slightly toxic to 
the cells or perhaps it doesn't like using up so much 
energy to express it), then some mutation may be selected 
for that prevents high expression.  I would be tempted to 
try starting from the very beginning with fresh plasmid 
containing your gene of interest.  Ideally, this plasmid 
will not have "lived" inside an E.coli cell too long and so 
won't have been altered itself.  Often, in expression 
systems, it is recommended to re-transform the E.coli that 
you will be using for your expression every time you get 
ready to grow up a batch.  All the best.

John Marcus

> Hi all,
> I'm a graduate student doing biophysics with a stronger background in
> physics than in bio-related areas.  Earlier this year we obtained an E.
> coli strain (SMH 50) which overexpresses adenylate kinase (AK).  My very
> first prep went very well - a yield of >70mg/L was obtained with
> excellent enzyme activity.  But, ever since then I have not been able to
> get consistant results in quantity and activity.  To make sure that the
> bacteria from a "strong and healthy" origin are picked before large
> scale amplification I've been measuring the ratio of Volume_Activity
> (U/ml) and OD at 280nm for crude extract of each 5ml overnight
> preculture.  This ratio varies in a wide range, eg. 5 ~ 100 U/OD.  What
> troubles me is if a saved culture sample (plated on agar dish, or with
> 15% glycerol in freezer or liquid nitrogen) with high Activity/OD is
> inoculated again the ratio may turn out to be much lower after overnight
> shaking.  I'd like to know if this Activity/OD is the best measure of
> the efficiency of protein expression in bacteria cells.  What are the
> possible reasons for low and/or inconsistant expression?
> Many thanks for your help.

John Marcus            Marcus at tpp.uq.edu.au (Dr J.Marcus)
Cooperative Research Centre for Tropical Plant Pathology
5th Level John Hines Building
University of Queensland
St. Lucia, QLD 4072

Fax: 61-7-3365-4771
Phone: 61-7-3365-4764

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