Abstract
The chlorosomes of green sulfur bacteria (GSB) are mainly assembled from one of three types of bacteriochlorophylls (BChls), BChls c, d, and e. By analogy to the relationship between BChl c and BChl d (20-desmethyl-BChl c), a fourth type of BChl, BChl f (20-desmethyl-BChl e), should exist but has not yet been observed in nature. The bchU gene (bacteriochlorophyllide C-20 methyltransferase) of the brown-colored green sulfur bacterium Chlorobaculum limnaeum was inactivated by conjugative transfer from Eshcerichia coli and homologous recombination of a suicide plasmid carrying a portion of the bchU. The resulting bchU mutant was greenish brown in color and synthesized BChl fF. The chlorosomes of the bchU mutant had similar size and polypeptide composition as those of the wild type (WT), but the Qy absorption band of the BChl f aggregates was blue-shifted 16 nm (705 nm vs. 721 nm for the WT). Fluorescence spectroscopy showed that energy transfer to the baseplate was much less efficient in chlorosomes containing BChl f than in WT chlorosomes containing BChl e. When cells were grown at high irradiance with tungsten or fluorescent light, the WT and bchU mutant had identical growth rates. However, the WT grew about 40% faster than the bchU mutant at low irradiance (10 μmol photons m−2 s-1). Less efficient energy transfer from BChl f aggregates to BChl a in the baseplate, the much slower growth of the strain producing BChl f relative to the WT, and competition from other phototrophs, may explain why BChl f is not observed naturally.
Highlights
Chlorosomes are the defining property of green bacteria and are the light-harvesting structures used for phototrophic growth of these bacteria (Blankenship and Matsuura, 2003; Frigaard and Bryant, 2006; Oostergetel et al, 2010; Bryant et al, 2012)
Selected colonies were subjected to three rounds of restreaking on selective media, and individual transconjugant colonies were screened by polymerase chain reaction (PCR) amplification of a fragment spanning the bchU gene and the inserted plasmid
We report here the construction of a bchU mutant in C. limnaeum, the first targeted mutation constructed in a browncolored green sulfur bacteria (GSB)
Summary
Chlorosomes are the defining property of green bacteria and are the light-harvesting structures used for phototrophic growth of these bacteria (Blankenship and Matsuura, 2003; Frigaard and Bryant, 2006; Oostergetel et al, 2010; Bryant et al, 2012). A GSB cell contains ∼200 chlorosomes, and a green bacterial cell contains ∼50 million BChl molecules, which together account for ∼30% of the cellular carbon (Frigaard and Bryant, 2006). These enormous light-harvesting antennas allow green bacteria to grow at extremely low irradiances at which no other phototrophs can survive. Examples include GSB that grow at a depth of ∼110 meters in the Black Sea (Manske et al, 2005; Marschall et al, 2010) and a GSB that was isolated at a depth of ∼2200 m on the floor of the Pacific Ocean near a black smoker (Beatty et al, 2005)
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