A cDNA clone, pAgthi1, encoding a homologue of yeast Thi4, which is involved in thiazole biosynthesis, was isolated from a library made from poly(A) RNA from actinorhizal nodules of Alnus glutinosa by differential screening with nodule and root cDNA, respectively. The corresponding gene, agthi1, was shown to be expressed at high levels in nodules and shoot tips of A. glutinosa, while having low expression levels in roots, flowers, and developing fruits. The function of AgThi1 was demonstrated by yeast complementation studies, in which AgThi1 was able to rescue a yeast thi4 mutant when fused to the yeast Thi4 signal peptide. In A. glutinosa nodules, high levels of agthi1 mRNA were detected in the infected cortical cells and in the pericycle of the nodule vascular system. A homologue of this cDNA, ara6/tz, was identified in Arabidopsis thaliana. ara6 maps in a region of chromosome 5 of Arabidopsis containing the tz locus which is consistent with the fact that ara6 transcription is disturbed in two tz mutant lines. ara6/tz is expressed at high levels in chloroplast-containing parenchymatic cells of leaves, inflorescence shoots and flowers of Arabidopsis, and at lower levels in the vascular system.