B. pseudofirmus OF4, MotPS functions as the sole sodium-dependent stator for the
flagellar motor [3], while O. iheyensis also contains the proton-coupled MotAB
stator [15]. The difference between these species might be due to the more
halotolerant nature of the latter. Conversely, Bacillus clausii KSM-K16 contains
set genes with high homology to MotAB from B. subtilis that can couple both
sodium and protons for motility [53]. Another sodium channel, NaChBac, was
found to play a major role in the pH homeostasis of B. halodurans C-125 and
B. pseudofirmus OF4, particularly in conditions where sodium and solute concentrations are low [54, 55].
5 The Alkaliphilic Respiration Chain
As noted above, the bioenergetics of alkaliphiles depends on the delicate balance
between pH homeostasis and the PMF. The acidification of the cytoplasm has the
detrimental effect of significantly reducing the PMF across the membrane. For
instance, the PMF of the facultative alkaliphile Bacillus sp. TA2.A1 shifts from
À164 to À78 mV when pH is changed from 7.5 to 10 [56]. Despite this apparent
limitation, aerobic alkaliphiles are adapted to cope with a reduced proton electrochemical gradient [41]. Examples of high pH-specific adaptations include
the overexpression of cytochromes in some strains when grown in high pH
environments [57] and the fact that the redox midpoint potential of these cytochromes is markedly lower than for their neutrophilic counterparts, as is the case
with the cytochrome-b from Bacillus firmus RAB [58]. The respiratory chain of
B. pseudofirmus OF4 is similar to that in other Bacillus species, containing two
NADH dehydrogenases, a succinate dehydrogenase, as well as cytochrome bd and
cytochrome caa 3 . Other than the elevated cytochrome expression, the other components of the respiratory chain maintain constant expression levels across different
pHs [41].
The archaeon Natrialba magadii contains a full arsenal of proteins that allow for
efficient respiration and oxidative phosphorylation. These include the operon
atpHIKECFAB, encoding a proton-coupled ATP synthase, genes encoding for
type II and mitochondrial NADH dehydrogenases, as well as genes for the cytochrome c-type terminal oxidase subunits I and II and for the cytochrome ubiquinol
oxidase I on II. Conversely, the N. magadii genome does not encode a cytochrome
bc1, which is predicted to couple reduced quinone to the electron carrier halocyanin
in halophilic archaea. The presence of several two-component signal transduction
systems as well as three rhodopsin homologue genes and two loci encoding for
chemoreceptors suggests that N. magadii is capable of sensing and responding to a
variety of light and chemical signals [18].
Genomics of Alkaliphiles
147
flagellar motor [3], while O. iheyensis also contains the proton-coupled MotAB
stator [15]. The difference between these species might be due to the more
halotolerant nature of the latter. Conversely, Bacillus clausii KSM-K16 contains
set genes with high homology to MotAB from B. subtilis that can couple both
sodium and protons for motility [53]. Another sodium channel, NaChBac, was
found to play a major role in the pH homeostasis of B. halodurans C-125 and
B. pseudofirmus OF4, particularly in conditions where sodium and solute concentrations are low [54, 55].
5 The Alkaliphilic Respiration Chain
As noted above, the bioenergetics of alkaliphiles depends on the delicate balance
between pH homeostasis and the PMF. The acidification of the cytoplasm has the
detrimental effect of significantly reducing the PMF across the membrane. For
instance, the PMF of the facultative alkaliphile Bacillus sp. TA2.A1 shifts from
À164 to À78 mV when pH is changed from 7.5 to 10 [56]. Despite this apparent
limitation, aerobic alkaliphiles are adapted to cope with a reduced proton electrochemical gradient [41]. Examples of high pH-specific adaptations include
the overexpression of cytochromes in some strains when grown in high pH
environments [57] and the fact that the redox midpoint potential of these cytochromes is markedly lower than for their neutrophilic counterparts, as is the case
with the cytochrome-b from Bacillus firmus RAB [58]. The respiratory chain of
B. pseudofirmus OF4 is similar to that in other Bacillus species, containing two
NADH dehydrogenases, a succinate dehydrogenase, as well as cytochrome bd and
cytochrome caa 3 . Other than the elevated cytochrome expression, the other components of the respiratory chain maintain constant expression levels across different
pHs [41].
The archaeon Natrialba magadii contains a full arsenal of proteins that allow for
efficient respiration and oxidative phosphorylation. These include the operon
atpHIKECFAB, encoding a proton-coupled ATP synthase, genes encoding for
type II and mitochondrial NADH dehydrogenases, as well as genes for the cytochrome c-type terminal oxidase subunits I and II and for the cytochrome ubiquinol
oxidase I on II. Conversely, the N. magadii genome does not encode a cytochrome
bc1, which is predicted to couple reduced quinone to the electron carrier halocyanin
in halophilic archaea. The presence of several two-component signal transduction
systems as well as three rhodopsin homologue genes and two loci encoding for
chemoreceptors suggests that N. magadii is capable of sensing and responding to a
variety of light and chemical signals [18].
Genomics of Alkaliphiles
147