Rzeznicka Kamila, Walker Caroline J, Westergren Tomas, Kannangara C Gamini, von Wettstein Diter, Merchant Sabeeha, Gough Simon P, Hansson Mats
Department of Biochemistry, Lund University, Box 124, SE-22100 Lund, Sweden.
Proc Natl Acad Sci U S A. 2005 Apr 19;102(16):5886-91. doi: 10.1073/pnas.0501784102. Epub 2005 Apr 11.
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.
叶绿素生物合成是一个涉及约20个不同酶促步骤的过程。这些步骤中有一半是其他四吡咯生物合成（如血红素）所共有的。最不清楚的酶促步骤之一是异环的形成，这是所有（细菌）叶绿素分子的一个特征。在叶绿体中，异环的形成是由镁原卟啉IX单甲酯环化酶催化的需氧反应。需氧环化酶反应的体外测定需要叶绿体的膜结合成分和可溶性成分。来自大麦（Hordeum vulgare L.）Xantha-1和Viridis-k位点突变体的提取物没有环化酶活性。通过Percoll梯度离心对分离的质体进行分级分离表明，Xantha-1和Viridis-k突变体在与叶绿体膜相关的成分上存在缺陷。与拟南芥CHL27、嗜胶红环菌acsF、莱茵衣藻CRD1和CTH1相对应且位于大麦染色体3（3H）短臂上的Xantha-1基因被克隆，并对Xantha-1(35)、Xantha-1(81)和Xantha-1(82)中的突变进行了表征。这一发现将生化和遗传数据联系起来，因为它表明Xantha-1编码一个膜结合环化酶亚基。证据表明，需氧环化酶至少需要一种可溶性成分和两种膜结合成分。
