When you have an antibiotic resistance gene in a plasmid, how do ensure that the enzyme is constitutively expressed so that your selection always works?
As long as there is antibiotic around (and no resistance mutants pop up) the selection is self-supported. A bacteria that does not express the resistance protein will die...
Agree with Yoram. Additionally, if you are making a plasmid de novo, you need to make sure that a promoter is upstream the antibiotic resistance gene. In commercially available constructs e.g. pUC19 you don't have to worry about this as they have already been engineered with a promoter which will be constitutively active.
Yoram and Mark, thank you! I could not identify the promoter region in a plasmid that I am using (based on the plasmid map). That is why I wanted to get some more information.
The question of promoters, at least in E. coil, is not an simple one. You can try looking at those links:
http://margalit.huji.ac.il/promec/index.html
https://molbiol-tools.ca/Promoters.htm
And this paper
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0090447
Yoram
Yoram, I wasn't aware of these databases. Thank you for sharing :-)
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