Fig. (2).

The CRN regulating galactose uptake in yeast. The constitutively expressed transcription activator Gal4p binds to the promoters of GAL genes. In the absence of galactose, the represser Gal80p blocks the activation domain of Gal4p. In the presence of intracellular galactose. the signal transducer Gal3p is activated, which sequesters Gal80p in the cytoplasm, leading to transcription of the GAL genes. The signal transducer gal3 and the represser gal80 are basally expressed in non-induced, non-glucose-repressed conditions. Lines terminating in arrowheads denote positive regulation. Lines terminating in bars denote repression. Gene symbols denote transcription. For enzymatic interactions, the relevant enzymes arc indicated next to each line. The double ellipses denote Gal4p dimers. See main text for references.