Inactivation of bciD and bchU genes in the green sulfur bacterium Chlorobaculum limnaeum and alteration of photosynthetic pigments in the resultant mutants

• The bciD and bchU genes of Chlorobaculumn (Cba.) limnaeum were inactivated.
• The resultant mutant gave bacteriochlorophyll(BChl)-d as chlorosomal pigments.
• The mutation and light intensity affected the composition of photosynthetic pigments.
• The mutation using Cba. limnaeum RK-j-1 was capable of producing BChls-c, d, e and f.

The bciD and bchU genes were earlier identified as relating to the C7-formylation and to catalyzing the C20-methylation, respectively, of bacteriochlorophyll(BChl)-e biosynthesis in the green sulfur bacterium Chlorobaculum (Cba.) limnaeum. The resultant ΔbciD and ΔbchU mutants produced BChl-c and BChl-f, respectively, as the BChl pigments in the cultured cells. In this study, the mutant deleting both the genes was constructed and successfully accumulated BChl-d in the same species. The series of the mutants using the Cba. limnaeum RK-j-1 strain enabled us to synthesize the four different BChl pigments, BChls-c, d, e, and f, in the identical species. In order to evaluate photo-adaptation mechanisms of green sulfur bacteria, alteration of photosynthetic pigments including carotenoids (Cars) by the mutation as well as illuminated light intensity was investigated using HPLC. For the BChl pigments, inactivation of the bciD gene induced drastic changes in the composition of BChl homologs (further methylation at the 82-position from the 8-ethyl to isobutyl group) and epimers (inversion of the 31-stereochemistry from (31R)- to (31S)-configuration) compared to inactivation of the bchU gene; the total contents of the (31S)-epimers accounted for 23–36% in the wild type strain and for 87–90% in the ΔbciD/bchU double mutant, depending on illuminated light intensity. Such mutation did not affect Car biosynthesis, but the composition was strongly light-dependent. With an increase of illuminated light intensity, Cars possessing aromatic ϕ-end group(s), isorenieratene and β-isorenieratene that were characteristic of the Cba. limnaeum species, decreased for all three mutants as well as the wild type strain. Concomitantly, the contents of Cars possessing β-end groups, β-carotene and 7,8-dihydro-β-carotene, increased and were dominant under high-light illumination.

Figure optionsDownload as PowerPoint slideInactivation of bciD and bchU genes in the green sulfur bacterium Chlorobaculum limnaeum and alteration of photosynthetic pigments in the resultant mutants.