Abstract
Anoxygenicphotosynthetic prokaryotes have simplified photosystems that represent ancient lineages that predate the more complex oxygen evolving photosystems present in cyanobacteria and chloroplasts. These organisms thrive under illuminated anaerobic photosynthetic conditions, but also have the ability to grow under dark aerobic respiratory conditions. This study provides a detailed snapshot of transcription ground states of both dark aerobic and anaerobic photosynthetic growth modes in the purple photosynthetic bacterium Rhodobactercapsulatus. Using 18 biological replicates for aerobic and photosynthetic states, we observed that 1834 genes (53 % of the genome) exhibited altered expression between aerobic and anaerobic growth. In comparison with aerobically grown cells, photosynthetically grown anaerobic cells showed decreased transcription of genes for cobalamin biosynthesis (−45 %), iron transport and homeostasis (−42 %), motility (−32 %), and glycolysis (−34 %). Conversely and more intuitively, the expression of genes involved in carbon fixation (547 %), bacteriochlorophyll biosynthesis (162 %) and carotenogenesis (114 %) were induced. We also analysed the relative contributions of known global redox transcription factors RegA, FnrL and CrtJ in regulating aerobic and anaerobic growth. Approximately 50 % of differentially expressed genes (913 of 1834) were affected by a deletion of RegA, while 33 % (598 out of 1834) were affected by FnrL, and just 7 % (136 out of 1834) by CrtJ. Numerous genes were also shown to be controlled by more than one redox responding regulator.
Highlights
Purple anoxygenic photosynthetic bacteria are among the most diverse micro-organisms studied in regard to their ability to generate metabolic energy
This includes aerobic and anaerobic respiratory, chemoautotrophic, photoautotrophic and photoheterotrophic growth modes [1]. This growth versatility should promote Rhodobacter species as attractive organisms for the production of useful chemicals; little is known about the transcription changes that provide control in driving carbon metabolism or transcriptional allocation under different growth conditions. It is not known what percentage of the Rhodobacter capsulatus transcriptome is dedicated to biosynthesis of haem, cobalamin and bacteriochlorophyll under anaerobic photosynthetic conditions, or to homeostasis of iron and other metals under aerobic respiratory conditions
A P value 0.05 was used as a cut-off, with the results reported as log2 fold-changes in expression as defined as anaerobic photosynthetic expression divided by dark aerobic expression [10, 19]
Summary
Purple anoxygenic photosynthetic bacteria are among the most diverse micro-organisms studied in regard to their ability to generate metabolic energy. This includes aerobic and anaerobic respiratory, chemoautotrophic, photoautotrophic and photoheterotrophic growth modes [1]. This growth versatility should promote Rhodobacter species as attractive organisms for the production of useful chemicals; little is known about the transcription changes that provide control in driving carbon metabolism or transcriptional allocation under different growth conditions. Understanding regulatory changes in gene expression can provide a foundation to the metabolic changes that allow this genus to thrive under so many different environmental conditions. This type of analysis can provide an insight to how these cells can use transcription to control the flux of metabolic pathways that can lead to downstream applications in the production of useful chemicals such as hydrogen (H2) and polyhydroxybutyrate for renewable biofuel and biodegradable plastics, respectively [2,3,4,5]
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