Identification and characterization of four distinct asparagine synthetase (AsnS) genes in maize (Zea mays L.)

Four distinct full-length cDNA clones encoding asparagine synthetase (AS; EC 6.3.5.4) were isolated from maize (Zea mays). Detailed analysis of two these genes suggested the previously published maize AsnS gene (gi 984262) is possibly a chimera. In silico analysis identified additional members of this new monocot-specific class in rice genomic DNA and sorghum EST collections. The presence of these new AsnS genes suggests maize has a representative from each of the major classes (class I, class II, and class III). TaqMan® expression analysis indicates Zm-AsnS1 and Zm-AsnS4 are ubiquitously expressed in the tissue panel tested, Zm-AsnS3 is largely restricted to root and root crown tissues and Zm-AsnS2 is largely absent from above-ground green tissue. Expression of all four distinct AsnS genes in maize root tissues raises the possibility that earlier studies on the enzymology of AsnS in maize root tips reported results obtained from a pool of different AsnS activities in the root.