The Maize Oil Yellow1 (Oy1) Gene Encodes the I Subunit of Magnesium Chelatase

Abstract

Semi-dominant Oil yellow1 (Oy1) mutants of maize (Zea mays) are deficient in the conversion of protoporphyrin IX to magnesium protoporphyrin IX, the first committed step of chlorophyll biosynthesis. Using a candidate gene approach, a cDNA clone was isolated that was predicted to encode the I subunit of magnesium chelatase (ZmCHLI) and mapped to the same genetic interval as Oy1. Allelic variation was identified at ZmCHLI between wild-type plants and plants carrying semi-dominant alleles of Oy1. These differences revealed putative amino acid substitutions that could account for the alterations in protein function. Candidate lesions were tested by introduction of homologous changes into the Synechocystis magnesium chelatase I gene (SschlI) and characterization of the activity of mutant protein variants in an in vitro enzyme activity assay. The results of these analyses suggest that SsChlI protein variants containing the substitutions identified in the dominant Oy1 maize alleles lack activity necessary for magnesium chelation and confer a semi-dominant phenotype via competitive inhibition of wild-type SsChlI.

Inducible expression of magnesium protoporphyrin chelatase subunit I (CHLI)-amiRNA provides insights into cucumber mosaic virus Y satellite RNA-induced chlorosis symptoms

“…The present results are consistent with previous studies on the Y-sat-mediated pathogenesis of CMV [34,35]. Similar chlorosis phenotypes were also observed in Arabidopsis thaliana [30] and Zea mays [31] mutants, and in CHLI-silenced transgenic tobacco plants [26]. The present experimental system has an advantage over the virus-infected plants, mutants, and the constitutively silenced transgenic plants in that it enables us to analyze early phase of chlorosis development, as manifested by the decreased chlorophyll content, the downregulation of CPRGs expression, and the induction of defense-related genes at 2 dpt, when the plants had not shown any visible sign of chlorosis development.…”

“…Along with the induction of yellow phenotypes, a drastic decrease in CHLI mRNA level and a high-level accumulation of Y-sat derived siRNA were observed. Similar yellow phenotypes were observed in Arabidopsis thaliana (AtchlI) [30], Zea mays L. (ZmchlI) [31] mutants and CHLI-silenced transgenic tobacco plants [26].…”

Recessiveness and Dominance in Barley Mutants Deficient in Mg-Chelatase Subunit D, an AAA Protein Involved in Chlorophyll Biosynthesis

“…Previous analyses of semidominant and recessive barley and maize (Zea mays) Mg-chelatase mutants deficient in the I subunit have also provided knowledge on the interactions between the subunits (Hansson et al, , 2002Sawers et al, 2006). The D subunit could not be detected in any of the studied barley I mutants, whereas wild-type levels of the D subunit were found in barley mutants that lacked the H subunit (Lake et al, 2004).…”

“…Thus, the recessive xantha-h mutants contained no I protein, whereas wild-type levels were found in the semidominant mutants. For in vitro studies, the semidominant mutations found in barley and maize were constructed in existing expression systems for R. capsulatus bchI and Synechocystis chlI, respectively (Hansson et al, 2002;Sawers et al, 2006). The resulting I proteins showed no or reduced ATPase activity and could not contribute to the Mg-chelatase activity.…”

Suppression of the Barleyuroporphyrinogen III synthaseGene by aDsActivation Tagging Element Generates Developmental Photosensitivity

“…Unlike the four preceding enzymatic steps ( Figure 1A), reduced Mg-chelatase activity does not cause photosensitivity, but rather reduced chlorophyll levels due to a negative feedback regulatory mechanism (Hansson et al, 1999(Hansson et al, , 2002Papenbrock et al, 2000aPapenbrock et al, , 2000bSoldatova et al, 2005;Sawers et al, 2006). The substrate of Mg-chelatase, protoporphyrin IX, is the branch point of chlorophyll and heme synthesis ( Figure 1A); by contrast, antisense suppression of the first committed heme synthetic enzyme, plastidic ferrochelatase, does lead to protoporphyrin IX accumulation and light-dependent necrosis (Papenbrock et al, 2001).…”