Maresca Julia A, Gomez Maqueo Chew Aline, Ponsatí Marta Ros, Frigaard Niels-Ulrik, Ormerod John G, Bryant Donald A
Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA 16802, USA.
J Bacteriol. 2004 May;186(9):2558-66. doi: 10.1128/JB.186.9.2558-2566.2004.
Bacteriochlorophylls (BChls) c and d, two of the major light-harvesting pigments in photosynthetic green sulfur bacteria, differ only by the presence of a methyl group at the C-20 methine bridge position in BChl c. A gene potentially encoding the C-20 methyltransferase, bchU, was identified by comparative analysis of the Chlorobium tepidum and Chloroflexus aurantiacus genome sequences. Homologs of this gene were amplified and sequenced from Chlorobium phaeobacteroides strain 1549, Chlorobium vibrioforme strain 8327d, and C. vibrioforme strain 8327c, which produce BChls e, d, and c, respectively. A single nucleotide insertion in the bchU gene of C. vibrioforme strain 8327d was found to cause a premature, in-frame stop codon and thus the formation of a truncated, nonfunctional gene product. The spontaneous mutant of this strain that produces BChl c (strain 8327c) has a second frameshift mutation that restores the correct reading frame in bchU. The bchU gene was inactivated in C. tepidum, a BChl c-producing species, and the resulting mutant produced only BChl d. Growth rate measurements showed that BChl c- and d-producing strains of the same organism (C. tepidum or C. vibrioforme) have similar growth rates at high and intermediate light intensities but that strains producing BChl c grow faster than those with BChl d at low light intensities. Thus, the bchU gene encodes the C-20 methyltransferase for BChl c biosynthesis in Chlorobium species, and methylation at the C-20 position to produce BChl c rather than BChl d confers a significant competitive advantage to green sulfur bacteria living at limiting red and near-infrared light intensities.
细菌叶绿素（BChls）c和d是光合绿硫细菌中两种主要的捕光色素，它们的区别仅在于BChl c的C-20次甲基桥位置上存在一个甲基。通过对嗜热绿菌（Chlorobium tepidum）和橙黄嗜热栖热菌（Chloroflexus aurantiacus）基因组序列的比较分析，鉴定出一个可能编码C-20甲基转移酶的基因bchU。从分别产生BChls e、d和c的嗜盐绿菌（Chlorobium phaeobacteroides）菌株1549、弧状绿菌（Chlorobium vibrioforme）菌株8327d和弧状绿菌菌株8327c中扩增并测序了该基因的同源物。发现弧状绿菌菌株8327d的bchU基因中有一个单核苷酸插入，导致提前出现框内终止密码子，从而形成截短的、无功能的基因产物。该菌株产生BChl c的自发突变体（菌株8327c）有第二个移码突变，可恢复bchU中的正确阅读框。在产BChl c的嗜热绿菌中使bchU基因失活，所得突变体仅产生BChl d。生长速率测量表明，同一生物体（嗜热绿菌或弧状绿菌）中产BChl c和d的菌株在高和中等光照强度下具有相似的生长速率，但在低光照强度下，产BChl c的菌株比产BChl d的菌株生长得更快。因此，bchU基因编码嗜盐绿菌属中BChl c生物合成的C-20甲基转移酶，在C-20位置甲基化以产生BChl c而非BChl d，赋予了生活在有限红光和近红外光强度下的绿硫细菌显著的竞争优势。
