Identification of the Xan-g gene and expression of the Mg-chelatase encoding genes Xan-f, -g and -h in mutant and wild type barley (Hordeum vulgare L.)

In barley the loci Xantha-f, -g and -h (Xan-f, -g, -h) encode the three sub units of magnesium chelatase (Mg-chelatase). Mg-chelatase catalyzes the ins ertion of Mg2+ into protoporphyrin IX, the first step in the chlorophyll sp ecific branch of tetrapyrrole biosynthesis. Previously, the Xan-h and -f ge nes have been cloned and shown to encode Mg-chelatase subunits. In the pres ent study, a partial cDNA of the Xan-g gene has been isolated and shown to encode the third Mg-chelatase subunit. Southern blot analysis indicates tha t the Xan-g and -h genes are present as single copy genes in the barley gen ome. In addition, Western blot analysis of the three subunits in greening s eedlings shows that while the amount of the XAN-G and -H subunit protein ar e unchanged or only slightly increased, the protoporphyrin IX binding subun it, XAN-F, is light-inducible. Implications of this expression pattern are discussed. Furthermore, it is demonstrated that mutants of the Xan-h locus, which are deficient in the Xan-h transcript and corresponding 42-kDa prote in. in addition are deficient in the ca. 100 kDa XAN-G protein subunit, eve n though the Xan-g transcript is present in detectable amounts. It is sugge sted, that the stability of the XAN-G protein is affected by the presence o f the XAN-H subunit protein.