Production of phenotypically epimeraseless yeast by L-arabinose

The previous report from this laboratory that l-arabinose is a gratuitous inducer of the galactose transport system has been found to be an artefact resulting from the combination of galactose contamination of commercial samples of l-arabinose and inhibition by l-arabinose of galactose metabolism by inactivation of uridine-diphosphate-glucose-4-epimerase. As a result of l-arabinose inhibition of the metabolism of the contaminating galactose, galactose itself serves as a gratuitous inducer, producing phenotypically epimeraseless yeast.