Transcriptional responses to redox stress in Clostridium thermocellum

Redox stress and imbalance are likely contributing factors to Clostridium thermocellum metabolic inefficiencies affecting ethanol yield and titer.  C. thermocellum was grown in continuous culture with added hydrogen peroxide and methyl viologen, to final concentrations of 0.29 mM and 0.58 mM respectively, to better understand redox stress.  Samples were taken and analyzed during stressor concentration build-up and during steady states. Microarray analyses conducted on redox stressed cultures elucidated the global transcriptional responses of C. thermocellum to these two different redox conditions.  We observed differences in culture turbidity/cell yield, redox potential, and end-product formation during redox stress.  Redox potential gradually fell 44 mV (from -501 to -545 mV) in response to the methyl viologen addition and increased 24 mV (from -489 to -465 mV) in response to H₂0₂ addition.  Ethanol/Acetate ratio was nearly halved 11 hours after starting to relieve the methyl viologen stress.  In response to applied stresses, we observe differential expression for genes involved in sulfur metabolism and regulation, nitrogen and amino acid metabolism and membrane transport.  Our results suggest these gene products are key elements facilitating the cellular response to the redox stress conditions, which may be useful for future studies to redirect electron and carbon flow in C. thermocellum.  Increased transcription of genes involved in phage biosynthesis (a putative ~30 gene ‘phage island’) was observed in one replicate fermenter, indicative of possible prophage induction.  Follow-up phage induction experiments are being conducted and will be reported.