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Mutational analysis of three bchH paralogs in (bacterio-)chlorophyll biosynthesis in Chlorobaculum tepidum
Authors:Aline Gomez Maqueo Chew  Niels-Ulrik Frigaard  Donald A. Bryant
Affiliation:(1) Department of Biochemistry and Molecular Biology, The Pennsylvania State University, S-235 Frear Building, PA 16802 University Park, USA;(2) Department of Biology, University of Copenhagen, Copenhagen, Denmark;(3) Present address: Department of Microbiology, The Ohio State University, 43210 Columbus, OH, USA
Abstract:The first committed step in the biosynthesis of (bacterio-)chlorophyll is the insertion of Mg2+ into protoporphyrin IX by Mg-chelatase. In all known (B)Chl-synthesizing organisms, Mg-chelatase is encoded by three genes that are homologous to bchH, bchD, and bchI of Rhodobacter spp. The genomes of all sequenced strains of green sulfur bacteria (Chlorobi) encode multiple bchH paralogs, and in the genome of Chlorobaculum tepidum, there are three bchH paralogs, denoted CT1295 (bchT), CT1955 (bchS), and CT1957 (bchH). Cba. tepidum mutants lacking one or two of these paralogs were constructed and characterized. All of the mutants lacking only one of these BchH homologs, as well as bchS bchT and bchH bchT double mutants, which can only produce BchH or BchS, respectively, were viable. However, attempts to construct a bchH bchS double mutant, in which only BchT was functional, were consistently unsuccessful. This result suggested that BchT alone is unable to support the minimal (B)Chl synthesis requirements of cells required for viability. The pigment compositions of the various mutant strains varied significantly. The BChl c content of the bchS mutant was only ~10% of that of the wild type, and this mutant excreted large amounts of protoporphyrin IX into the growth medium. The observed differences in BChl c production of the mutant strains were consistent with the hypothesis that the three BchH homologs function in end product regulation and/or substrate channeling of intermediates in the BChl c biosynthetic pathway. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.
Keywords:Chlorophyll  Bacteriochlorophyll  Mg-chelatase  Protoporphyrin IX   Chlorobaculum tepidum   Green sulfur bacteria
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