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

Abstract

ABSTRACT Bacteriochlorophyll (BChl) c is the major photosynthetic pigment in the green sulfur bacterium Chlorobaculum tepidum , in which it forms protein-independent aggregates that function in light harvesting. BChls c , d , and e are found only in chlorosome-producing bacteria and are unique among chlorophylls because of methylations that occur at the C-8 2 and C-12 1 carbons. Two genes required for these methylation reactions were identified and designated bchQ (CT1777) and bchR (CT1320). BchQ and BchR are members of the radical S -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 , and bchQ bchR mutants of C. tepidum were constructed and characterized. The bchQ mutant produced BChl c that was not methylated at C-8 2 , the bchR mutant produced BChl c that was not methylated at C-12 1 , and the double mutant produced [8-ethyl, 12-methyl]-BChl c that lacked methylation at both the C-8 2 and C-12 1 positions. Compared to the wild type, the Q y absorption bands for BChl c in the mutant cells were narrower and blue shifted to various extents. All three mutants grew slower and had a lower cellular BChl c content than the wild type, an effect that was especially pronounced at low light intensities. These observations show that the C-8 2 and C-12 1 methylations of BChl c play important roles in the adaptation of C. tepidum to low light intensity. The data additionally suggest that these methylations also directly or indirectly affect the regulation of the BChl c biosynthetic pathway.