title: 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 approximately 20 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 membrane-bound 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.
authors.affiliation: Department of Biochemistry, Lund University, Box 124, SE-22100 Lund, Sweden.
authors.forename: Kamila
authors.initials: K
authors.lastname: Rzeznicka
authors.forename: Caroline J
authors.initials: CJ
authors.lastname: Walker
authors.forename: Tomas
authors.initials: T
authors.lastname: Westergren
authors.forename: C Gamini
authors.initials: CG
authors.lastname: Kannangara
authors.forename: Diter
authors.initials: D
authors.lastname: von Wettstein
authors.forename: Sabeeha
authors.initials: S
authors.lastname: Merchant
authors.forename: Simon P
authors.initials: SP
authors.lastname: Gough
authors.forename: Mats
authors.initials: M
authors.lastname: Hansson
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D002734:Chlorophyll / Q000096:biosynthesis*; D002736:Chloroplasts / Q000737:chemistry; D001467:Hordeum / Q000737:chemistry / Q000201:enzymology* / Q000235:genetics; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D015394:Molecular Structure; D010641:Phenotype; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D021122:Protein Subunits / Q000235:genetics / Q000378:metabolism*; D011524:Protoporphyrins / Q000737:chemistry* / Q000378:metabolism; D016415:Sequence Alignment; D013347:Subcellular Fractions / Q000737:chemistry / Q000201:enzymology
chemical_list: D008565:Membrane Proteins; D010940:Plant Proteins; D021122:Protein Subunits; D011524:Protoporphyrins; D002734:Chlorophyll; C033434:protoporphyrin IX monomethyl ester