Structure of the gene encoding nitrilase 1 from Arabidopsis thaliana . The nitrilases of Arabidopsis thaliana ( At ) catalyze the conversion of indole-3 - acetonitrile ( IAN ) to indole-3 - acetic acid ( IAA ) , thus controlling the last step of auxin biosynthesis . A full - length genomic clone encoding the complete cluster of the At nitrilases 1 to 3 ( NIT1-3 ) , including the respective promoter regions , has been isolated and the NIT1 isoform has been sequenced . The coding region ( nit1 ) spans about 2.3 kb and is composed of five exons separated by four introns . The exon - intron splice junctions agree with the consensus sequences typical for plant genes . In agreement with the known cDNA sequence , the exons encode a protein of 346 amino acids ( aa ) with a deduced molecular mass of 38.2 kDa. The transcription start point ( tsp ) of nit1 was determined by primer extension experiments . This tsp defines a 5' untranslated region of 36 bp and is located 32 bp downstream from a TATA box . The promoter region of nit1 is located within the approx . 1.5 - kb intergenic part that separates the nit2 and nit1 coding sections .