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 subunits of magnesium chelatase (Mg-chelatase). Mg-chelatase catalyzes the insertion of Mg2+ into protoporphyrin IX, the first step in the chlorophyll specific branch of tetrapyrrole biosynthesis. Previously, the Xan-g and -h genes have been cloned and shown to encode Mg-chelatase subunits. In the present 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 that the Xan-g and -It genes are present as single copy genes in the barley genome. In addition. Western blot analysis of the three subunits in greening seedlings shows that while the amount of the XAN-G and -H subunit protein are unchanged or only slightly increased, the protoporphyrin IX binding subunit, 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 protein, in addition are deficient in the ca. 100 kDa XAN-G protein subunit, even though the Xun-g transcript is present in detectable amounts. It is suggested, that the stability of the XAN-G protein is affected by the presence of the XAN-H subunit protein.