Bacteriochlorophyllide c C-8 2 and C-12 1 Methyltransferases Are Essential for Adaptation to Low Light in Chlorobaculum tepidum

Bacteriochlorophyll (BChl)c is the major photosynthetic pigment in the green sulfur bacteriumChlorobaculum tepidum , in which it forms protein-independent aggregates that function in light harvesting. BChlsc ,d , ande are found only in chlorosome-producing bacteria and are unique among chlorophylls because of methylations that occur at the C-82 and C-121 carbons. Two genes required for these methylation reactions were identified and designatedbchQ (CT1777) andbchR (CT1320). BchQ and BchR are members of the radicalS -adenosylmethionine (SAM) protein superfamily; each has sequence motifs to ligate a [4Fe-4S] cluster, and we propose that they catalyze the methyl group transfers.bchQ ,bchR , andbchQ bchR mutants ofC. tepidum were constructed and characterized. ThebchQ mutant produced BChlc that was not methylated at C-82 , thebchR mutant produced BChlc that was not methylated at C-121 , and the double mutant produced [8-ethyl, 12-methyl]-BChlc that lacked methylation at both the C-82 and C-121 positions. Compared to the wild type, the Qy absorption bands for BChlc in the mutant cells were narrower and blue shifted to various extents. All three mutants grew slower and had a lower cellular BChlc content than the wild type, an effect that was especially pronounced at low light intensities. These observations show that the C-82 and C-121 methylations of BChlc play important roles in the adaptation ofC. tepidum to low light intensity. The data additionally suggest that these methylations also directly or indirectly affect the regulation of the BChlc biosynthetic pathway.