Plasmid-Stability? Fragment-Deletion? I'm in dire need to identify and solve - (Oct/19/2010 )
TGS on Wed Oct 27 15:45:40 2010 said:
Nice find! From the paper I understand in smaller plasmids a part of Z-DNA forming inserts is recombined (even in RECA1 genotype- DH1) after incubation in bioreactor, but unaltered plasmids remain. I wonder what happens for plasmids of larger size. Perhaps there is just a small part left that is unchanged. Still, it's strange that he can't even find a few clones with the expected plasmid.
I'd guess as soon as a smaller plasmid with the resistence is formed it will have a major advantage since it means less metabolic stress to reproduce and also allows faster reproduction. This way it becomes the dominant form in a colony.
One of my latter expression showed something what could be a trace of a higher band (even in digestion) at the expected size. I recently made a gel with lots amount of plasmid of two of these samples and cut them out. I'm thinking about retransforming them and prepping them just to see if these were different Iso-forms or if in this case some of the original plasmid remained... but I'm rather pessimistic.
To me it seems also strange that all clones are negative ...if it is due to recombination it must be a really nasty sequence feature!
The only thing to elucidate if it is really due to recombination would be sequencing of the plasmid and look for the exact deletion events and check if they are flanked by some repeats or Z-DNA stretches ...but that's laborious and does not really help you in the course of your project since you are looking for functional plasmids
Another thing that comes to my mind is:
Maybe what you get is a combination of some recombination and low efficiency in ligation? ...you are cloning long fragments of DNA (10kb) ...what ligase ara you using? Normal T4 Ligase is not very efficient in cloning large fragments.
Maybe you can use a ligase that is known for good efficiency with large fragments like this one.
Maybe this one does the trick
Best regards,
p
The only thing to elucidate if it is really due to recombination would be sequencing of the plasmid and look for the exact deletion events and check if they are flanked by some repeats or Z-DNA stretches ...but that's laborious and does not really help you in the course of your project since you are looking for functional plasmids
Another thing that comes to my mind is:
Maybe what you get is a combination of some recombination and low efficiency in ligation? ...you are cloning long fragments of DNA (10kb) ...what ligase ara you using? Normal T4 Ligase is not very efficient in cloning large fragments.
Maybe you can use a ligase that is known for good efficiency with large fragments like this one.
Maybe this one does the trick
Best regards,
p
What bothers me most is that here in my lab this plasmid was already several times succesfully mutated (even though other mutations were done), so it seems it's not a general problem of this plasmid, although one can see that in the older mutation experiments also some plasmids appeared which shared characteristics with my problem (a short fragment which fits fine and another fragment which is way smaller than it should be).
I really tried a lot of things yet like skipping dialysis, trying other cells (XL1 Blue instead of TOP10) incubating with lower amounts of antibiotics, incubating at 30°C and also picking a plate completely empty... all which yielded no results... the next and probably last thing I'll try (at least my other projects start to running more smoothly) is using NEB10beta cells and using heat shock for transformation. Since I always good good amounts of colonies I'm optimistic that the lower transformation-efficiency won't really bother me...
P.S.:
Last week I created two plasmids (2.5 and 3 kb in pET41a -> 7.5 and 8 kB) on the first try... so it seems two be really a specific problem of these constructs and rather not of myself or the materials I use.
Never thought that the problem is personally associated with you
Sometimes things tourn out to be tricky ...but it is essential to keep going on ...no matter if several experiments fail ...it is still biology ...and in minor cases personal failure!
Keep on rocking
...i think you are on the right way!Regards,
p
-pDNA-