A cDNA clone encoding Arabidopsis thaliana galactokinase was fortuitously isolated during the course of a screen for plant homologues of a Saccharomyces cerevisiae peroxisome assembly gene, PAS9. Clones were sought which restored the ability of pas9 cells to grow on oleate as a sole carbon source, as oleate metabolism requires peroxisomal beta-oxidation and therefore functional peroxisomes. Subsequent experiments showed that high level expression of the galactokinase cDNA did not complement the peroxisomal assembly defect, but instead permitted the cells to grow on agar plates in the absence of an external carbon source. Agar plates were shown to contain a small amount of galactose released from the agar as a result of autoclaving. The galactokinase clone was shown to be functional, as it could complement a S. cerevisiae galactokinase mutant. Galactokinase is a single copy gene in Arabidopsis, which has been designated AGK1, and is expressed in all the major organs of the plant.