8
1.7 Role of Iron in Fungal Physiology
Iron is the fourth abundant element in our planet next to oxygen, silicon, and aluminum. It is an essential element in almost all living beings except the Lactobacillus
genus. It is an indispensable component for microbial growth and development.
Various biological processes such as the synthesis of DNA are contributing to
endogenous enzyme activity (Hofte 1992; Winkelmann 2007). The environmental
iron exists in both reduced (Fe
2+
) and oxidized forms (Fe
3+
). The ability to donate or
accept electrons between the reduced and oxidized forms enables iron to play an
integral role in the biological reactions (Neilands 1981). The iron required by the
fungi is not readily available in nature. Several fungal species have established various mechanisms to acquire iron. The release of siderophores and the organic acid
surge are the two main strategies employed by fungi for ensuring the availability of
iron for their physiological processes (Perez- Miranda et al. 2007; Karuna et al.
2010). The dependency of fungi on iron to execute their normal physiological functions makes iron as an important limiting factor for their growth.
The fungi depend on iron for various metabolic activities including redox-active
reactions, absorption and active uptake of iron, translocation, solubilization/weathering, and biogenic mineral production (Dave et al. 2006). During the redox-active
reactions, insoluble Fe
3+
ion is converted to soluble Fe
2+
ion. Multiple mechanisms
are proposed for the procurement of iron absorption and active uptake by fungi. The
most common ones are (i) the facilitated uptake by iron-chelating agents like siderophores, (ii) reductive iron assimilation, (iii) acquiring ferrous iron with less pull, and
(iv) acquiring iron from heme and its subsequent breakdown. Candida spp. that are
pathogenic, Cryptococcus neoformans that causes opportunistic infections, and
Saccharomyces cerevisiae that is extensively beneficial in food production do not
produce siderophores. Instead, these microorganisms utilize the iron chelated by
other siderophore-producing organisms through various mechanisms such as reductive iron assimilation, heme uptake, and low-affinity iron uptake to acquire iron. The
morphology of most of the microorganisms that do not produce siderophore resembles the morphology of yeast. It is interesting to note that certain yeast species like
Schizosaccharomyces pombe or Aureobasidium pullulans produce siderophores. The
reductive iron assimilation is absent in species like Aspergillus nidulans. Iron toxicity
and deficiency are controlled by cells through a defined iron regulatory mechanism.
1.8 Iron Sensing and Transcriptional Regulation
S. cerevisiae, a poor prototype of many fungal species, is extensively studied for its
iron homeostasis function. C. glabrata has the iron-regulating mechanism similar to
S. cerevisiae. Other fungal species matching the iron-regulating mechanism of
S. cerevisiae are unknown. In S. cerevisiae and C. glabrata, an Aft transcription
activator upregulates the genes involved in iron uptake under the iron-deprived environment. The process is achieved by the binding of Fe-S clusters present in the
S. S. Arputhanantham et al.
1.7 Role of Iron in Fungal Physiology
Iron is the fourth abundant element in our planet next to oxygen, silicon, and aluminum. It is an essential element in almost all living beings except the Lactobacillus
genus. It is an indispensable component for microbial growth and development.
Various biological processes such as the synthesis of DNA are contributing to
endogenous enzyme activity (Hofte 1992; Winkelmann 2007). The environmental
iron exists in both reduced (Fe
2+
) and oxidized forms (Fe
3+
). The ability to donate or
accept electrons between the reduced and oxidized forms enables iron to play an
integral role in the biological reactions (Neilands 1981). The iron required by the
fungi is not readily available in nature. Several fungal species have established various mechanisms to acquire iron. The release of siderophores and the organic acid
surge are the two main strategies employed by fungi for ensuring the availability of
iron for their physiological processes (Perez- Miranda et al. 2007; Karuna et al.
2010). The dependency of fungi on iron to execute their normal physiological functions makes iron as an important limiting factor for their growth.
The fungi depend on iron for various metabolic activities including redox-active
reactions, absorption and active uptake of iron, translocation, solubilization/weathering, and biogenic mineral production (Dave et al. 2006). During the redox-active
reactions, insoluble Fe
3+
ion is converted to soluble Fe
2+
ion. Multiple mechanisms
are proposed for the procurement of iron absorption and active uptake by fungi. The
most common ones are (i) the facilitated uptake by iron-chelating agents like siderophores, (ii) reductive iron assimilation, (iii) acquiring ferrous iron with less pull, and
(iv) acquiring iron from heme and its subsequent breakdown. Candida spp. that are
pathogenic, Cryptococcus neoformans that causes opportunistic infections, and
Saccharomyces cerevisiae that is extensively beneficial in food production do not
produce siderophores. Instead, these microorganisms utilize the iron chelated by
other siderophore-producing organisms through various mechanisms such as reductive iron assimilation, heme uptake, and low-affinity iron uptake to acquire iron. The
morphology of most of the microorganisms that do not produce siderophore resembles the morphology of yeast. It is interesting to note that certain yeast species like
Schizosaccharomyces pombe or Aureobasidium pullulans produce siderophores. The
reductive iron assimilation is absent in species like Aspergillus nidulans. Iron toxicity
and deficiency are controlled by cells through a defined iron regulatory mechanism.
1.8 Iron Sensing and Transcriptional Regulation
S. cerevisiae, a poor prototype of many fungal species, is extensively studied for its
iron homeostasis function. C. glabrata has the iron-regulating mechanism similar to
S. cerevisiae. Other fungal species matching the iron-regulating mechanism of
S. cerevisiae are unknown. In S. cerevisiae and C. glabrata, an Aft transcription
activator upregulates the genes involved in iron uptake under the iron-deprived environment. The process is achieved by the binding of Fe-S clusters present in the
S. S. Arputhanantham et al.