The Gal7p galactose-1-phosphate uridylyltransferase catalyzes the conversion of galactose-1-phosphate to UDP-galactose, a key step in galactose catabolism (4, 5). The enzyme is found in the cytoplasm and functions as a dimer (3, 6). When galactose is the sole carbon source, gal7 null mutants are unable to grow and accumulate high levels of galactose-1-phosphate (5).

All the galactose structural genes (GAL1, GAL10, GAL7, GAL2) are coordinately regulated at the level of transcription in response to galactose by Gal4p, Gal80p, and Gal3p (4, 7, and reviewed in 8). Regardless of carbon source, the Gal4p transcriptional activator is bound as a dimer to upstream activation sites found in the promoters of the GAL genes. In the presence of galactose, Gal3p sequesters the transcriptional repressor Gal80p in the cytoplasm, thereby relieving inhibition of Gal4p and resulting in GAL gene expression (9). In the absence of galactose, Gal80p remains bound as a dimer, to Gal4p, preventing Gal4p from recruiting other factors of the Pol II transcription machinery (reviewed in 8).

Galactose-1-phosphate uridylyltransferase is conserved from E. coli to man (10). Mutations in human GALT (OMIM), the ortholog of yeast Gal7p, have been associated with the potentially lethal disease classic galactosemia (OMIM). Patients must restrict dietary intake of galactose, but even with this provision they can suffer complications that include learning disabilities and speech/motor dysfunction (11 and references therein).

Last updated: 2006-10-12