Vogl Kajetan, Tank Marcus, Orf Gregory S, Blankenship Robert E, Bryant Donald A
Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park PA, USA.
Front Microbiol. 2012 Aug 10;3:298. doi: 10.3389/fmicb.2012.00298. eCollection 2012.
The chlorosomes of green sulfur bacteria (GSB) are mainly assembled from one of three types of bacteriochlorophylls (BChls), BChls c, d, and e. By analogy to the relationship between BChl c and BChl d (20-desmethyl-BChl c), a fourth type of BChl, BChl f (20-desmethyl-BChl e), should exist but has not yet been observed in nature. The bchU gene (bacteriochlorophyllide C-20 methyltransferase) of the brown-colored green sulfur bacterium Chlorobaculum limnaeum was inactivated by conjugative transfer from Eshcerichia coli and homologous recombination of a suicide plasmid carrying a portion of the bchU. The resulting bchU mutant was greenish brown in color and synthesized BChl f(F). The chlorosomes of the bchU mutant had similar size and polypeptide composition as those of the wild type (WT), but the Q(y) absorption band of the BChl f aggregates was blue-shifted 16 nm (705 nm vs. 721 nm for the WT). Fluorescence spectroscopy showed that energy transfer to the baseplate was much less efficient in chlorosomes containing BChl f than in WT chlorosomes containing BChl e. When cells were grown at high irradiance with tungsten or fluorescent light, the WT and bchU mutant had identical growth rates. However, the WT grew about 40% faster than the bchU mutant at low irradiance (10 μmol photons m(-2) s(-1)). Less efficient energy transfer from BChl f aggregates to BChl a in the baseplate, the much slower growth of the strain producing BChl f relative to the WT, and competition from other phototrophs, may explain why BChl f is not observed naturally.
绿硫细菌（GSB）的叶绿体主要由三种细菌叶绿素（BChls）之一组装而成，即BChls c、d和e。根据BChl c与BChl d（20-去甲基-BChl c）之间的关系类推，应该存在第四种类型的BChl，即BChl f（20-去甲基-BChl e），但在自然界中尚未观察到。通过大肠杆菌的接合转移以及携带部分bchU的自杀质粒的同源重组，使褐色绿硫细菌嗜泥绿杆菌的bchU基因（细菌叶绿素ide C-20甲基转移酶）失活。所得的bchU突变体呈绿褐色，并合成了BChl f（F）。bchU突变体的叶绿体与野生型（WT）的叶绿体具有相似的大小和多肽组成，但BChl f聚集体的Q（y）吸收带蓝移了16 nm（WT为721 nm，突变体为705 nm）。荧光光谱表明，与含有BChl e的野生型叶绿体相比，含有BChl f的叶绿体中向基板的能量转移效率要低得多。当细胞在钨丝灯或荧光灯下高辐照度下生长时，WT和bchU突变体具有相同的生长速率。然而，在低辐照度（10 μmol光子 m(-2) s(-1)）下，WT的生长速度比bchU突变体快约40%。BChl f聚集体向基板中BChl a的能量转移效率较低、产生BChl f的菌株相对于WT生长慢得多以及来自其他光合生物的竞争，可能解释了为什么在自然界中未观察到BChl f。
