The apt gene, coding for adenine phosphoribosyltransferase (APRT), has been isolated from the plant Arabidopsis thaliana. Data from both Southern analysis and characterization of apt clones isolated from a genomic library is consistent with the occurrence of one apt within the A. thaliana genome. Comparison of the nucleotide sequence of the apt gene with its corresponding cDNA indicates that the gene contains five introns, whereas all other apt isolated to date have fewer introns (four in mammals, two in Drosophila). The locations of the introns within the plant apt coding region are not consistent with the placement of introns in the previously isolated apt of murines, human and Drosophila species. In agreement with its expression pattern in vivo, the upstream region of this plant apt is able to express the beta-glucuronidase-encoding gene (gus) in an apparently constitutive manner in transgenic A. thaliana plants. The apt promoter region is notable for its lack of conventional promoter elements such as TATA, CCAAT or G+C-rich sequence elements.