synthesis. Moreover, it has been observed that ATP synthase expression increases
when microbes such as B. subtilis, Corynebacterium glutamicum, E. coli,
Desulfovibrio vulgaris, etc. are subjected to alkaline treatment [102, 148, 165–
167]. ATP synthase synthesizes ATP when H
+
flows from cell surface to cytoplasm
through it, and this contributes to the intracellular H
+ concentration. Hence, it is
expected contributing to the pH homeostasis process. In fact, results of a mutational
study reflect the pH homeostasis role played by ATP synthase. As aforementioned,
Mrp antiporter is important in translocating H
+ from the extracellular environment to
the cytoplasm and known to play a vital role in pH homeostasis. An Mrp antiporter
deletion mutant of B. subtilis exhibited a rise in ATP synthase expression [168]. The
rise in the level of synthase may compensate the loss in H
+ translocation due to the
Mrp deletion. This also indicates that the bioenergetics and pH homeostasis processes are wired tightly.
Although high level ATP synthase expression is widely accepted and experimentally supported as means of high pH adaptation, it seems that the case is not
universal. The transcriptome analysis of alkaline-stressed Enterococcus faecalis
revealed that the ATP synthase was significantly downregulated when the cells
were grown in pH 10 media [169]. The authors’ findings also include a significant
drop in the expression of the nhaC gene which encodes Na
+
/H
+ antiporter. This
also contradicts to the main stream notion that recognizes enhanced expression of
the antiporter at high pH. However, the authors did not mention how these
downregulations help the organism to survive the alkaline condition. However, it
is tempting to speculate that E. faecalis is a lactic acid bacterium and in the presence
of glucose, which the authors added in the medium they used, can produce acid, and
this can possibly maintain low intracellular pH when grown in alkaline media. Thus,
it is possible that in order to survive in the alkaline condition, the cells shift their
metabolism to produce more acid and which may reduce the need for OXPHOSbased ATP synthase. However, this needs further studies. For instance, what will the
transcriptome trend show if a non-fermentable medium is used?
Adaptations of the a-Subunit
ATP synthase is a multicomponent enzyme. One of these components believed to be
involved in high pH adaptation is the a-subunit. Alignment studies on the a-subunit
amino acid sequences of alkaliphilic and non-alkaliphilic Bacillus species revealed
that the transmembrane helix-4 (TMH4) and transmembrane helix-5 (TMH5) are
somehow distinct between these two groups of bacteria. The TMH4 of alkaliphiles
has a conserved motif of 171MRxxxxVxxKxxxM, while TMH5 has two conserved
residues, L205 and G212 [95, 170]. The fair conservation of these residues only
among sequences of alkaliphiles suggests their possible role in high pH adaptation.
Mutational studies on the conserved residues were done to elucidate their adaptive
roles. Based on the analysis of the mutants, it seems that residues V177 and K180 are
involved in H
+ uptake pathway [171]. Similarly, M171, M184, I185, and L205 are
also believed to be relevant to the a-subunit proton pathway [95]. These authors also
Challenges and Adaptations of Life in Alkaline Habitats
107
when microbes such as B. subtilis, Corynebacterium glutamicum, E. coli,
Desulfovibrio vulgaris, etc. are subjected to alkaline treatment [102, 148, 165–
167]. ATP synthase synthesizes ATP when H
+
flows from cell surface to cytoplasm
through it, and this contributes to the intracellular H
+ concentration. Hence, it is
expected contributing to the pH homeostasis process. In fact, results of a mutational
study reflect the pH homeostasis role played by ATP synthase. As aforementioned,
Mrp antiporter is important in translocating H
+ from the extracellular environment to
the cytoplasm and known to play a vital role in pH homeostasis. An Mrp antiporter
deletion mutant of B. subtilis exhibited a rise in ATP synthase expression [168]. The
rise in the level of synthase may compensate the loss in H
+ translocation due to the
Mrp deletion. This also indicates that the bioenergetics and pH homeostasis processes are wired tightly.
Although high level ATP synthase expression is widely accepted and experimentally supported as means of high pH adaptation, it seems that the case is not
universal. The transcriptome analysis of alkaline-stressed Enterococcus faecalis
revealed that the ATP synthase was significantly downregulated when the cells
were grown in pH 10 media [169]. The authors’ findings also include a significant
drop in the expression of the nhaC gene which encodes Na
+
/H
+ antiporter. This
also contradicts to the main stream notion that recognizes enhanced expression of
the antiporter at high pH. However, the authors did not mention how these
downregulations help the organism to survive the alkaline condition. However, it
is tempting to speculate that E. faecalis is a lactic acid bacterium and in the presence
of glucose, which the authors added in the medium they used, can produce acid, and
this can possibly maintain low intracellular pH when grown in alkaline media. Thus,
it is possible that in order to survive in the alkaline condition, the cells shift their
metabolism to produce more acid and which may reduce the need for OXPHOSbased ATP synthase. However, this needs further studies. For instance, what will the
transcriptome trend show if a non-fermentable medium is used?
Adaptations of the a-Subunit
ATP synthase is a multicomponent enzyme. One of these components believed to be
involved in high pH adaptation is the a-subunit. Alignment studies on the a-subunit
amino acid sequences of alkaliphilic and non-alkaliphilic Bacillus species revealed
that the transmembrane helix-4 (TMH4) and transmembrane helix-5 (TMH5) are
somehow distinct between these two groups of bacteria. The TMH4 of alkaliphiles
has a conserved motif of 171MRxxxxVxxKxxxM, while TMH5 has two conserved
residues, L205 and G212 [95, 170]. The fair conservation of these residues only
among sequences of alkaliphiles suggests their possible role in high pH adaptation.
Mutational studies on the conserved residues were done to elucidate their adaptive
roles. Based on the analysis of the mutants, it seems that residues V177 and K180 are
involved in H
+ uptake pathway [171]. Similarly, M171, M184, I185, and L205 are
also believed to be relevant to the a-subunit proton pathway [95]. These authors also
Challenges and Adaptations of Life in Alkaline Habitats
107