Xantha-l encodes a membrane subunit of the aerobic Mg-protoporphyrin IX monomethyl ester cyclase involved in chlorophyll biosynthesis

Abstract

Chlorophyll biosynthesis is a process involving Ϸ20 different enzymatic steps. Half of these steps are common to the biosynthesis of other tetrapyrroles, such as heme. One of the least understood enzymatic steps is formation of the isocyclic ring, which is a characteristic feature of all (bacterio)chlorophyll molecules. In chloroplasts, formation of the isocyclic ring is an aerobic reaction catalyzed by Mg-protoporphyrin IX monomethyl ester cyclase. An in vitro assay for the aerobic cyclase reaction required membranebound and soluble components from the chloroplasts. Extracts from barley (Hordeum vulgare L.) mutants at the Xantha-l and Viridis-k loci showed no cyclase activity. Fractionation of isolated plastids by Percoll gradient centrifugation showed that xantha-l and viridis-k mutants are defective in components associated with chloroplast membranes. The Xantha-l gene, corresponding to Arabidopsis thaliana CHL27, Rubrivivax gelatinosus acsF, Chlamydomonas reinhardtii CRD1, and CTH1 and situated at the short arm of barley chromosome 3 (3H), was cloned, and the mutations in xantha-l 35 , xantha-l 81 , and xantha-l 82 were characterized. This finding connected biochemical and genetic data because it demonstrated that Xantha-l encodes a membrane-bound cyclase subunit. The evidence suggests that the aerobic cyclase requires at least one soluble and two membrane-bound components.barley ͉ acsF ͉ CHL27 ͉ CRD1 ͉ mutant

Three classes of oxygen-dependent cyclase involved in chlorophyll and bacteriochlorophyll biosynthesis

“…It has long been believed that the O 2 -dependent cyclase reaction requires NAD(P)H as a reductant (5), with dependence observed in plant, algal, and cyanobacterial systems (20,40). The barley xantha-l and viridis-k mutants that accumulate MgPME are deficient in the membrane components of the O 2 -dependent enzyme, with all xantha-l mutations mapping to the single acsF ortholog that is intact in the viridis-k mutants (11). Subsequently, Bollivar et al (41) concluded that viridis-k mutants do not lack Ycf54; thus, it is possible that the missing subunit of the enzyme, disrupted in viridis-k mutants, is a membrane-associated NAD(P)H-binding protein.…”

“…gelatinosus contains both BchE and AcsF cyclases, conferring the ability to synthesize BChl under varying O 2 regimes (9). Orthologs of acsF are widely distributed in phototrophs and have been studied in higher plants (10,11), algae (12) and cyanobacteria (13), the green nonsulfur bacterium Chloroflexus aurantiacus (14), and the purple alphaproteobacterium Rhodobacter (Rba.) sphaeroides (15) (Fig.…”

Conserved Chloroplast Open-reading Frame ycf54 Is Required for Activity of the Magnesium Protoporphyrin Monomethylester Oxidative Cyclase in Synechocystis PCC 6803

“…In the anaerobic cyclase the 13 1 -oxo group of bacteriochlorophyll is derived from water (13), whereas in aerobic phototrophs such as Roseobacter denitrificans this group arises from molecular oxygen (14). Partial purification of oxidative cyclase from chloroplasts and cyanobacteria suggested that this enzyme requires several proteins (subunits) for activity (6,15). The only known candidate for a catalytic cyclase subunit was identified by mutational analysis of the photosynthetic bacterium Rubrivivax gelatinosus.…”

“…Thus, R. gelatinosus possesses two unrelated MgPME cyclases, one oxygen-dependent and one oxygen-independent (16,17). AcsF homologs have since been identified in many photosynthetic eukaryotes including C. reinhardtii (18), Arabidopsis thaliana (19), and barley (15). Two acsF-like genes, sll1214 and sll1874, have been identified in Synechocystis PCC 6803 (hereafter Synechocystis) (20); Sll1214 is essential for growth under aerobic conditions (20) and, to a certain extent, micro-oxic conditions (21), whereas Sll1874 was found to be essential for growth in micro-oxic conditions (20,21).…”

Responses of barley Albina and Xantha mutants deficient in magnesium chelatase to soil salinity

“…The Albina and Xantha barley plants carrying a mutation preventing chloroplast development are suitable for analysis of the roles of chloroplast in salt stress response. Although these mutations are lethal, the large seed endosperm can support plant growth for several weeks, allowing studies on these mutants (Dal Bosco et al 2003, Olsson et al 2004, Rzeznicka et al 2005, Svensson et al 2006, Campoli et al 2009, Müller and Hansson 2009. To investigate the roles of chloroplast development in formation of salt tolerance in barley, the nonallelic Albina and Xantha mutants were exposed to soil salinity (200 mmol) for 6 days and the stomatal performance, Na + /K + homeostasis and ROS metabolism were analysed.…”