STRUCTURAL-ANALYSIS OF THE NIT2 NIT1/NIT3 GENE-CLUSTER ENCODING NITRILASES, ENZYMES CATALYZING THE TERMINAL ACTIVATION STEP IN INDOLE-ACETIC ACID BIOSYNTHESIS IN ARABIDOPSIS-THALIANA/

A 13.8 kb DNA sequence containing the promoters and the structural gen es of the Arabidopsis thaliana nit2/nit1/nit3 gene cluster has been is olated and characterized. The coding regions of nit2, nit1 and nit3 sp anned 1.9, 1.8 and 2.1 kb, respectively. The architecture of the three genes is highly conserved. Each isoform consists of five exons separa ted by four introns. The introns are very similar with respect to size and position, but differ considerably in sequence composition. In con trast to the coding sequences the three promoters are very different i n sequence, size and in their repertoire of cis elements, suggesting d ifferential regulation of the three nitrilase isoenzymes by the develo pmental program of the plant and by diverse environmental factors. The nit1 promoter was subjected to analysis in planta. Translational fusi ons placing the nit1 full-length promoter and a series of 5'-deletion fragments in front of the uidA gene encoding beta-glucuronidase (GUS) were used for Agrobacterium tumefaciens-mediated transformation of Nic otiana tabacum. GUS expression was highest in fully expanded leaves an d in the shoot apex as well as in the apices of developing lateral bud s, whereas the GUS activity displayed by developing younger leaflets w as restricted to the tips of the expanding leaves. Within the root tis sue GUS expression was restricted to the root tips and the tips of new ly forming lateral roots. Structural features of the nitrilase gene fa mily and nitrilase gene expression patterns are discussed in context w ith current knowledge of auxin biosynthesis and auxin effects on diffe rent tissues.