Which prevents the lac genes in the dna of e. coli from being expressed most of the time?

The Lac repressor prevents the Lac genes in the DNA of E.coli from being expressed most of the time. The Lac repressor is actually a kind of DNA binding protein. It is a fact that in case there is an absence of lactose, the Lac repressor binds to the proteins. I hope that this answer comes to your help.

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Long term continuous groundwater monitoring, especially with warning systems and anthropic activity monitors