IDENTIFICATION OF AGTHI1, WHOSE PRODUCT IS INVOLVED IN BIOSYNTHESIS OF THE THIAMINE PRECURSOR THIAZOLE, IN ACTINORHIZAL NODULES OF ALNUS-GLUTINOSA

A cDNA clone, pAgthi1, encoding a homologue of yeast Thi4 which is inv olved in thiazole biosynthesis, was isolated from a library made from poly(A) RNA from actinorhizal nodules of Alnus glutinosa by differenti al screening with nodule and root cDNA, respectively. The correspondin g gene, agthi1, was shown to be expressed at high levels in nodules an d shoot tips of A. glutinosa, while having low expression levels in ro ots, flowers, and developing fruits. The function of AgThi1 was demons trated by yeast complementation studies, in which AgThi1 was able to r escue 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 vascula r system. A homologue of this cDNA, ara6/tz, was identified in Arabido psis thaliana, ara6 maps in a region of chromosome 5 of Arabidopsis co ntaining the tz locus which is consistent with the fact that ara6 tran scription 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 i n the vascular system.