What is a regulable promoter? And how does one regulate it? I'm reading a patent where they (in S. cerevisae YNP5 strain): > downregulate the ERG9 gene by replacing the native ERG9 promoter with the regulable MET3 promoter What's a regulable promoter and how does one regulate it? Must one add something to the broth to down-regulate the expression? I don't see anything special in the culture medium.

Regulatable promoter means that the expression of the gene downstream of this promoter can be either induced or repressed. Met3 is an example of such regulatable promoters and it is regulated by the addition of methionine to the media. In yeast, Met3 gene encodes for ATP sulphurylase, an enzyme in the methionine biosynthetic pathway. Upon the addition of methionine, the gene driven by Met3 promoter is repressed, as its expression level is reversely correlated with the concentration of methionine added to the media (Figure from this paper):

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