In the past few years, the activity of the metalloproteinase expressed by bacteria was very low, but recently several expressions have suddenly shown an increase in activity. The used Plasmid vector, E. coli strain, expression conditions, and purification methods have not changed, do not know why? Is it possible to mutate?
If you are not reviving the strain from the same glycerol stock it is possible that mutations would accumulate over prolonged culture. A more likely cause is batch to batch variation in the media ingredients. Have you recently opened a new container of something used in the media?
You must be growing your E. coli in something or on plates. That media is made out of compounds and crude ingredients which can be subject to decomposition or batch to batch variation (especially for plant/animal products). If you think it is a mutation just extract the plasmid and have it sequenced.
Robert Adolf Brinzer is trying to tell you the difference may be simply in expression level, rather then the gene sequence. This is commonly due to variation between lots of growth media, especially if you make your own media from ingredients in a recipe.
Another possibility is that your E coli may have mutated, rather than the plasmid. Again, as Robert Adolf Brinzer indicated, this won't be a problem if you freeze down many aliquots of your stock plasmid and host, so that you're always using the same starting material.
Thanks for the new explanation. The culture environment is basically the same. How does a bacterial mutation, but not a plasmid mutation, cause an increase in the activity of the expressed protein? Unfortunately, the initial plasmid has been used up. Can I assume that this is a new mutant of the protein?
Could be in the coding regions or promoters of enzymes involved in amino acid synthesis, tRNAs, protein degradation pathways, ribosomal proteins, or transcription factors that is leading to an upregulation of the expression of your gene rather than an increase in the activity which could be caused by kinases. Have you quantified the amount of your protein? If you have a glycerol stock of the original strain you can grow some and extract the initial plasmid. You cannot assume it is a mutation in the CDS of your protein as it could also be in the promoter.
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