Figure 1.

Schematic diagram of the GAL regulatory network in S. Cerevisiæ. Galactose import is guaranteed by Gal2p; internalized galactose activates Gal3p, which sequesters Gal80p in the cytoplasm, shuttling it from the nucleus. The transcriptional activator Gal4p, which is constitutively bound to promoters of the GAL genes, is then released from the inhibitory action of Gal80p and activates expression of the GAL1, GAL2, GAL3 and GAL80 genes. The regulatory network features two positive and one negative feedback loops.