https://www.wikiwand.com/en/Regulation_of_gene_expression
See more details at Gene regulatory network
From GEB
to prevent the relevant enzymes from ever being manufactured!
How does a cell stop a gene from being expressed? The answer is, it prevents it from ever getting transcribed. This means that it has to prevent RNA polymerase from doing its job. This can be accomplished by placing a huge obstacle in its path, along the DNA. precisely in front of that gene which the cell wants not to get transcribed. Such obstacles do exist, and are called repressors. They are themselves proteins, and they bind to special obstacle-holding sites on the DNA, called (I am not sure why) operators. An operator therefore is a site of control for the gene (or genes) which immediately follow it: those genes are called its operon. Because a series of enzymes often act in concert in carrying out a long chemical transformation, they are often coded for in sequence; and this is why operons often contain several genes, rather than just one. The effect of the successful repression of an operon is that a whole series of genes is prevented from being transcribed, which means that a whole set of related enzymes remains unsynthesized.
What about positive feedback and feedforward? Here again, there are two options: (1) unclog the clogged enzymes, or (2) stop the repression of the relevant operon. (Notice how nature seems to love double-negations! Probably there is some very deep reason for this.)
The mechanism by which repression is repressed involves a class of molecules called inducers. The role of an inducer is simple: it combines with a repressor protein before the latter has had a chance to bind to an operator on a DNA molecule; the resulting "repressor- inducer complex" is incapable of binding to an operator, and this leaves the door open for the associated operon to be transcribed into mRNA and subsequently translated into protein. Often the end product or some precursor of the end product can act as an inducer.