Xu Hong, Vavilin Dmitrii, Vermaas Wim
Department of Plant Biology and Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, Arizona 85287-1601, USA.
J Biol Chem. 2002 Nov 8;277(45):42726-32. doi: 10.1074/jbc.M205237200. Epub 2002 Aug 30.
Both chlorophyll (Chl) a and b accumulate in the light in a Synechocystis sp. PCC 6803 strain that expresses higher plant genes coding for a light-harvesting complex II protein and Chl a oxygenase. This cyanobacterial strain also lacks photosystem (PS) I and cannot synthesize Chl in darkness because of the lack of chlL. When this PS I-less/chlL(-)/lhcb(+)/cao(+) strain was grown in darkness, small amounts of two unusual tetrapyrroles, protochlorophyllide (PChlide) b and pheophorbide (pheide) b, were identified. Accumulation of PChlide b trailed that of PChlide a by several days, suggesting that PChlide a is an inefficient substrate of Chl a oxygenase. The presence of pheide b in this organism suggests a breakdown of Chl b via a pathway that does not involve conversion to a-type pigments. When the PS I-less/chlL(-) control strain was grown in darkness, Chl degradation was much slower than in the PS I-less/chlL(-)/lhcb(+)/cao(+) strain, suggesting that the presence of Chl b leads to more rapid turnover of Chl-binding proteins and/or a more active Chl degradation pathway. Levels and biosynthesis kinetics of Chl and of its biosynthetic intermediates are very different in the PS I-less/chlL(-)/lhcb(+)/cao(+) strain versus in the control. Moreover, when grown in darkness for 14 days, upon the addition of delta-aminolevulinic acid, the level of magnesium-protoporphyrin IX increased 60-fold in the PS I-less/chlL(-)/lhcb(+)/cao(+) strain (only approximately 2-fold in the PS I-less/chlL(-) control strain), whereas the PChlide and protoheme levels remained fairly constant. We propose that a b-type PChlide, Chl, or pheide in the PS I-less/chlL(-)/lhcb(+)/cao(+) strain may bind to tetrapyrrole biosynthesis regulatory protein(s) (for example, the small Cab-like proteins) and thus affect the regulation of this pathway.
在表达编码捕光复合体II蛋白和叶绿素a加氧酶的高等植物基因的集胞藻PCC 6803菌株中，叶绿素（Chl）a和b在光照下都会积累。该蓝细菌菌株也缺乏光系统（PS）I，并且由于缺乏chlL，在黑暗中无法合成叶绿素。当这种无PS I/chlL(-)/lhcb(+)/cao(+)菌株在黑暗中生长时，鉴定出了少量两种不寻常的四吡咯，即原叶绿素酸酯（PChlide）b和脱镁叶绿素（pheide）b。PChlide b的积累比PChlide a滞后几天，这表明PChlide a是叶绿素a加氧酶的低效底物。该生物体中脱镁叶绿素b的存在表明叶绿素b通过一条不涉及转化为a型色素的途径分解。当无PS I/chlL(-)对照菌株在黑暗中生长时，叶绿素降解比无PS I/chlL(-)/lhcb(+)/cao(+)菌株慢得多，这表明叶绿素b的存在导致叶绿素结合蛋白的周转更快和/或叶绿素降解途径更活跃。无PS I/chlL(-)/lhcb(+)/cao(+)菌株与对照相比，叶绿素及其生物合成中间体的水平和生物合成动力学有很大差异。此外，当在黑暗中生长14天后添加δ-氨基乙酰丙酸时，无PS I/chlL(-)/lhcb(+)/cao(+)菌株中镁原卟啉IX的水平增加了60倍（无PS I/chlL(-)对照菌株中仅增加约2倍），而PChlide和原血红素水平保持相当恒定。我们提出，无PS I/chlL(-)/lhcb(+)/cao(+)菌株中的b型PChlide、叶绿素或脱镁叶绿素可能与四吡咯生物合成调节蛋白（例如，小的类Cab蛋白）结合，从而影响该途径的调节。
