TI - Isobutanol activations of PhoB and Fur are likely mediated by quinone function . AB - Owing to the profound effect isobutanol had on the respiratory system through quinone malfunction , we sought to identify additional TFs whose activity perturbations can be linked to quinone malfunction . We started by identifying additional two-component systems known to be regulated by quinones . Three other two-component systems are known to have quinone-related activation mechanisms . These include EvgAS of E.coli ( ubiquinone mediated ) , BvgAS of Bordetella pertussis ( ubiquinone mediated ) , and PhoPR of Bacillus subtillis ( menaquinol mediated ) ( Bock and Gross , 2002 ; Schau et al , 2004 ; Eldakak and Hulett , 2007 ) . EvgAS is homologous to BvgAS in structure , and the PhoBR system of E.coli is homologous to the PhoPR system of B.subtillis in structure and function ( BLAST results presented in Supplementary Table VI ) . Of these systems , PhoBR was identified as significantly perturbed by NCA . Although the EvgAS system is known to be regulated by quinones , EvgA was not identified as a significantly perturbed regulator by NCA ( P-value = 047 ) , and therefore was not investigated further in this study . To verify that PhoB was involved in the isobutanol response , transcriptome measurements were obtained from DeltaphoB treated with and without 1% isobutanol . Figure 5A lists the log10 expression ratios of PhoB-regulon members from wild-type and DeltaphoB experiments , along with an indication of whether the difference was significant at a P-value [?] 0.05 ( for exact P-values , see Supplementary Table VII ) . The majority of PhoB-regulon members perturbed by isobutanol in wild type ( nine out of 16 ) had significantly different expression ratios in DeltaphoB ( P-value [?] 0.05 ) . This provided additional evidence that PhoB is activated by isobutanol . Taken together with the knowledge that the PhoPR system of B.subtillis is homologous to the PhoBR system of E.coli and they regulate similar operons in response to the same stimulus ( phosphate starvation ) , we postulate that isobutanol disrupts the membrane and releases quinol inhibition of PhoR , which allows PhoR to autoPHOSphorylate and activate PhoB . To support this hypothesis , expression levels for PhoB target genes were measured under fermentative growth conditions using quantitative real-time PCR ( Materials and methods ) . As reduced Qs/QH2s are the primary form of quinone under fermentative conditions ( Shestopalov et al , 1997 ; Georgellis et al , 2001 ; Bekker et al , 2007 ) , our hypothesis would predict that unlike ArcA , isobutanol treatment should activate PhoB under fermentative conditions to a similar , if not stronger , extent than it does in an aerobic environment . Fermentative expression ratios are presented alongside their aerobic equivalents in Figure 4 . The genes phoB and pstS were selected as PhoB target genes because they were the lead genes in the significantly induced operons shown to be regulated by PhoB in DeltaphoB experiments . These results clearly show isobutanol activation of PhoB under fermentative and aerobic conditions is similar , and support a mechanism of PhoR inhibition by quinols and isobutanol-mediated PhoB activation through quinol malfunction . In addition to their regulatory role , quinones serve as important electron carriers for many respiratory processes . We reasoned that if isobutanol caused general Q/QH2 malfunction the activity of all membrane-bound enzymes that rely on Q/QH2 for their electron-carrier capabilities , including all cytochromes and both NADH dehydrogenases , would be disrupted . With this in mind , there may be TFs whose isobutanol response can be linked to quinone electron-transport malfunction . To identify these TFs , we identified all aerobic respiratory machineries known to utilize quinones when glucose is the sole carbon source . These include succinate dehydrogenase ( sdhCDAB ) , NADH dehydrogenase I ( nuo operon ) , NADH dehydrogenase II ( ndh ) and cytochrome bo3 ( cyoABCD ) . Malfunction of all of these complexes in E.coli would diminish the ability to convert NADH to NAD+ and generate PMF . It is also well documented that aerobic respiration generates superoxide ions ( O2 - ) , with NDH-II as the main generator of endogenous superoxide and NDH-I and SDH as smaller contributors ( Gennis and Stewart , 1996 ) . Therefore , malfunction of these enzymes would also decrease the O2 - level . Combined with an inability to convert NADH to NAD+ , a decrease in endogenous O2 - would cause reductive stress . Interestingly , it has been suggested that a reducing environment may activate Fur ( Jovanovic et al , 2006 ) , which is the most significantly perturbed regulator not associated with respiration in this study . Fur requires binding of Fe2+ to become active and repress genes of its regulon . It has been shown previously that O2 - deactivates Fur after its conversion to H2O2 by superoxide dismutases , through the Fenton reaction ( Blanchard et al , 2007 ) : Therefore , a decrease in endogenous O2 - generation would increase the availability of Fe2+ , through a decrease in H2O2 levels , and in effect activate Fur relative to the control . Indeed , on isobutanol stress , NCA deduced an increase in Fur activity . To verify that Fur was involved in the isobutanol response , transcriptome measurements were obtained from Deltafur treated with and without 1% isobutanol . Figure 5B lists the log10 expression ratios of Fur-regulon members from wild-type and Deltafur experiments , along with an indication of whether the difference was significant at a P-value [?] 0.05 ( for exact P-values , see Supplementary Table VII ) . The majority of Fur-regulon members perturbed by isobutanol in wild type ( 24 out of 40 ) had significantly different expression ratios in Deltafur ( P-value [?] 0.05 ) . This provided additional evidence that Fur is activated by isobutanol . We postulate that membrane disruption interferes with the ability of quinones to act as electron carriers , and thereby inhibits the enzymatic activity of NDH-I , NDH-II and SDH . Inhibition of these complexes results in reductive stress through a decrease in endogenous O2 - production and an increase in the NADH/NAD+ . This results in Fur activation through a diminished Fenton reaction and stabilization of Fe2+ . Once active , Fur represses the expression of genes related to iron homeostasis . To support the involvement of quinones in this hypothesis , expression levels for Fur target genes were measured under fermentative growth conditions using quantitative real-time PCR ( Materials and methods ) . Fermentative expression ratios are presented alongside their aerobic equivalents in Figure 4 . The genes fhuF and fecI were selected as Fur target genes because fhuF was the strongly repressed gene singly regulated by Fur , fecI is an iron-associated sigma factor , and they were both shown to be regulated by Fur in Deltafur experiments . The average expression changes of fhuF , an fecI , in response to isobutanol under fermentative conditions ( red bars ) are negligible compared with their expression changes in an aerobic environment ( blue bars ) . The observed fhuF activation under fermentative conditions ( average ~25-fold ) is in contrast to the strong aerobic repression of fhuF ( average ~17-fold ) . These results show that in the absence of respiration isobutanol fails to perturb the expression of Fur target genes to the same degree or direction observed under aerobic conditions , and supports a mechanism of isobutanol-mediated Fur activation through quinones malfunction .