Supressor strains and overexpression (fwd)

Robert Preston rapr at MED.PITT.EDU
Tue May 25 09:42:41 EST 1993

Forwarded message:
> 	The question is: if I use a bacterial strain with a supressor
> (e.g. SupB,C,D,E,F), will it supress the appropriate stop codons in an
> protein overexpression vector?  Or is it somehow specific for phage?  I
> noticed that most commericial expression vectors (e.g. pGEX) don't use the
> _amber_ stop codon, which is commonly supressed in many bacterial strains.
> T.B. Shin
> tshin at

I think a stop codon is a stop codon, suppressible (i.e., suppressed) wherever 
the appropriate suppressor is in the genetic background.  My question is, so
what?  Only a small fraction of the stops in question actually do get the
suppressor amino acid (otherwise, suppression would be lethal, since every
protein would be trashed).  So, assuming there really were a suppressor still
present in your strain background, you'd lose only a very small amount of
your overexpressed protein due to that mechanism.  It seems likely to me, too,
that strains would need special care to maintain a suppressor.  At least with
yeast, suppressed strains are necessarily more-or-less sickly (ea small 
amount of random stopping isn't optimal for growth) so the strains mutate to
lose the suppressor, unless there is some specific selection for suppression.
That is, I'd guess that "commonly supressed" strains may well have lost their
suppressor unless someone has been taking care of the suppressor mutation. 

Robert Preston
UPMC Pathology
rapr at

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