<jats:title>ABSTRACT</jats:title> <jats:p> Bacteriochlorophyll (BChl) <jats:italic>c</jats:italic> is the major photosynthetic pigment in the green sulfur bacterium <jats:italic>Chlorobaculum tepidum</jats:italic> , in which it forms protein-independent aggregates that function in light harvesting. BChls <jats:italic>c</jats:italic> , <jats:italic>d</jats:italic> , and <jats:italic>e</jats:italic> are found only in chlorosome-producing bacteria and are unique among chlorophylls because of methylations that occur at the C-8 <jats:sup>2</jats:sup> and C-12 <jats:sup>1</jats:sup> carbons. Two genes required for these methylation reactions were identified and designated <jats:italic>bchQ</jats:italic> (CT1777) and <jats:italic>bchR</jats:italic> (CT1320). BchQ and BchR are members of the radical <jats:italic>S</jats:italic> -adenosylmethionine (SAM) protein superfamily; each has sequence motifs to ligate a [4Fe-4S] cluster, and we propose that they catalyze the methyl group transfers. <jats:italic>bchQ</jats:italic> , <jats:italic>bchR</jats:italic> , and <jats:italic>bchQ bchR</jats:italic> mutants of <jats:italic>C. tepidum</jats:italic> were constructed and characterized. The <jats:italic>bchQ</jats:italic> mutant produced BChl <jats:italic>c</jats:italic> that was not methylated at C-8 <jats:sup>2</jats:sup> , the <jats:italic>bchR</jats:italic> mutant produced BChl <jats:italic>c</jats:italic> that was not methylated at C-12 <jats:sup>1</jats:sup> , and the double mutant produced [8-ethyl, 12-methyl]-BChl <jats:italic>c</jats:italic> that lacked methylation at both the C-8 <jats:sup>2</jats:sup> and C-12 <jats:sup>1</jats:sup> positions. Compared to the wild type, the Q <jats:sub>y</jats:sub> absorption bands for BChl <jats:italic>c</jats:italic> in the mutant cells were narrower and blue shifted to various extents. All three mutants grew slower and had a lower cellular BChl <jats:italic>c</jats:italic> content than the wild type, an effect that was especially pronounced at low light intensities. These observations show that the C-8 <jats:sup>2</jats:sup> and C-12 <jats:sup>1</jats:sup> methylations of BChl <jats:italic>c</jats:italic> play important roles in the adaptation of <jats:italic>C. tepidum</jats:italic> to low light intensity. The data additionally suggest that these methylations also directly or indirectly affect the regulation of the BChl <jats:italic>c</jats:italic> biosynthetic pathway.