GluTR2 Complements a hema1 Mutant Lacking Glutamyl-tRNA Reductase 1, but is Differently Regulated at the Post-Translational Level

Arabidopsis HEMA1 and HEMA2 encode glutamyl-tRNA reductase (GluTR) 1 and 2, the two isoforms of the initial enzyme of tetrapyrrole biosynthesis. HEMA1 is dominantly expressed in photosynthetic tissue, while HEMA2 shows low constitutive expression and is induced upon stress treatments. We introduce a new HEMA1 knockout mutant which grows only heterotrophically on MS (Murashige and Skoog) medium at low light, indicating that the remaining GluTR2 does not sufficiently compensate for the extensive needs of metabolic precursors for Chl. While hema1 accumulates low amounts of Chl, it contains more than half of the wild-type heme content. The functional diversity of the two GluTR isoforms was analyzed by means of complementation studies of the hema1 mutant by expression of pHEMA1::HEMA2 and p35S::HEMA1, respectively. Expression of both transgenes complements hema1, indicating that GluTR2 can likewise be involved in the synthesis of Chl and is not exclusively assigned to heme synthesis. In comparison with p35S::HEMA1-complemented hema1 and the wild type, GluTR2 expression under control of the HEMA1 promoter (pHEMA1) in pHEMA1::HEMA2-complemented hema1 mutants causes elevated protochlorophyllide levels under extended dark periods as well as in short-day-grown adult plants, resulting in the formation of necrotic leaf tissue. Although both GluTR isoforms have similar activity and contribute to 5-aminolevulinic acid synthesis for adequate accumulation of Chl and heme, it is proposed that the two proteins experience a different post-translational control in darkness and light. While GluTR2 continues 5-aminolevulinic acid synthesis in darkness, GluTR1 is efficiently inactivated by the interaction with the FLU (FLUORESCENT) protein, thereby preventing an accumulation of protochlorophyllide.