Cloning two genes for nicotianamine aminotransferase, a critical enzyme iniron acquisition (strategy II) in graminaceous plants

Nicotianamine aminotransferase (NAAT), the key enzyme involved in the biosy nthesis of mugineic acid family phytosiderophores (MAs), catalyzes the amin o transfer of nicotianamine (NA). MAs are found only in graminaceous plants , although NA has been detected in every plant so far investigated. Therefo re, this amino transfer reaction is the first step in the unique biosynthes is of MAs that has evolved in graminaceous plants. NAAT activity is dramati cally induced by Fe deficiency and suppressed by re resupply. Based on the protein sequence of NAAT purified from Fe-deficient barley (Hordeum vulgare ) roots, two distinct cDNA clones encoding NAAT, naat-A and naat-B, were id entified. Their deduced amino acid sequences were homologous to several ami notransferases, and shared consensus sequences for the pyridoxal phosphate- binding site lysine residue and its surrounding residues. The expression of both naat-A and naat-B is increased in Fe-deficient barley roots, while na at-B has a low level of constitutive expression in Fe-sufficient barley roo ts. No detectable mRNA from either naat-A or naat-B was present in the leav es of either Fe-deficient or Fe-sufficient barley. One genomic clone with a tandem array of naat-1 and naat-A in this order was identified. naat-B and naat-A each have six introns at the same locations. The isolation of NAAT genes will pave the way to understanding the mechanism of the response to F e in graminaceous plants, and may lead to the development of cultivars tole rant to Fe deficiency that can grow in calcareous soils.