Genome-wide gene expression analysis of the switch between acidogenesis and solventogenesis in continuous cultures of Clostridium acetobutylicum.

Clostridium acetobutylicum is able to switch from acidogenic growth to solventogenic growth. We used phosphate-limited continuous cultures that established acidogenic growth at pH 5.8 and solventogenic growth at pH 4.5. These cultures allowed a detailed transcriptomic study of the switch from acidogenesis to solventogenesis that is not superimposed by sporulation and other growth phase-dependent parameters. These experiments led to new insights into the physiological role of several genes involved in solvent formation. The adc gene for acetone decarboxylase is upregulated well before the rest of the sol locus during the switch, and pyruvate decarboxylase is induced exclusively for the period of this switch. The aldehyde-alcohol dehydrogenase gene adhE1 located in the sol operon is regulated antagonistically to the paralog adhE2 that is expressed during acidogenic conditions. A similar antagonistic pattern can be seen with the two paralogs of thiolase genes, thlA and thlB. Interestingly, the genes coding for the putative cellulosome in C. acetobutylicum are exclusively transcribed throughout solventogenic growth. The genes for stress response are only induced during the shift but not in the course of solventogenesis when butanol is present in the culture. Finally, the data clearly indicate that solventogenesis is independent from sporulation.

Essential role of the hprK gene in Ralstonia eutropha H16.

Ralstonia eutropha H16 possesses an incomplete phosphoenolpyruvate (PEP):sugar phosphotransferase system (PTS) composed of EI, HPr, EIIA(Ntr) (PtsN) and EIIA(Man) (PtsM). We could show that in vitro the incomplete PTS phosphorylation cascade is partially functional. HPr becomes phosphorylated by PEP and EI, and transfers the phosphoryl group to EIIA(Ntr), but only extremely slowly to EIIA(Man). Components of this system have previously been shown to regulate the metabolism of polyhydroxybutyrate. Downstream from ptsN this organism contains an hprK gene, which codes for a homologue of HPr kinase/phosphorylase. We show that this enzyme phosphorylates HPr using ATP as phosphoryl donor. Interestingly, hprK appeared to be essential in R. eutropha because this gene could not be deleted in the wild-type strain, but could be deleted in mutants lacking ptsH or ptsI. This suggests that an increase in the HPr and/or P approximate His-HPr concentrations might be responsible for the growth defect. To test this hypothesis, various ptsH alleles were introduced into the ptsH hprK double mutant. Complementation of this mutant was possible only with the ptsH(His15Ala) allele, but not with the wild-type or ptsH(Ser46Ala) alleles. We conclude that elevated amounts of His-15-phosphorylated HPr, formed in the hprK mutant, are responsible for its growth defect.