Thermoacidophile
A thermoacidophile is an
An apparent tradeoff has been described between
Unique Biological Adaptations
Chromosome Structure and Epigenetic Modifications
Most thermoacidophiles are archaeal, with Crenarchaea belonging to the Sulfolobales order serving as an model system. Many of the biological mechanisms used by Crenarchae are shared by all archaea but there are some lineage-specific differences unique to thermoacidophilic archaea. One example of the unique differences for thermoacidophillic archaea is their lack of eukaryote-like histones, which are typically involved in the packaging and reorganization of the chromosome, all Thermoplasma and Crenarchaeota lack histone like proteins.[7]
Thermoacidophillic archaea typically have single, small, circular chromosome between 1.5 and 3 Mbp in length.[8]Chromatin proteins are used to condense and organize the genome, however this is not done with histone-orthologs as in eukaryotes or some bacteria, a large evolutionary divergence that characterizes thermoacidophillic archaea. Instead of using histones, a type of protein called nucleoid associated proteins (NAPs) are expressed, however the degree of conservation between species varies from protein to protein. These proteins are typically between 7 and 10 kDa in size, are basic, and account for up to 5% of cellular protein, making them one of the most highly expressed proteins in the cell.[7]
The functionality of NAPs can be altered via post-translational modifications (PTMs). These epigenetic modifications have significant impacts on their functionality and the fitness of thermophiles in extreme environments. Methylation, a form of PTM , is a common alteration to NAP structure. It has been linked to the thermostabilization of the proteins as well as the regulation of genes in epigenetic studies.[9] An example of the impact of methylation can be seen in an adaptive laboratory evolution experiment, in which a strain of Sa. solfataricu developed a super-acid resistant phenotype, even though its genome had not changed from the reference sequence. Investigation revealed that the acid resistance was conferred by a difference in the methylation of Sso7d and Cren7, both Naps.[9] This highlights how a difference in methylation can have a significant impact on the fitness of thermophiles.
In addition to local structuring by NAPs, the Sulfolobales genome is also globally compartmentalized into two sub-Mbp compartments. Each compartment, often referred to as the A and B compartment respectively, is populated by specific sets of genes that vary in their level of transcription.[10] The A compartment generally contains genes involved in essential biological processes such as the creation of metabolic proteins, which are highly expressed in the cell. The B compartment holds genes related to environmental stress responses, CRISPR-Cas clusters and fatty acid metabolism. The exact mechanism regarding the global restructuring of the genome is not currently known, however a protein known as coalescin has been found to play a critical role in the restructuring, with an inverse correlation observed between the occurrence of coalescin and the transcriptional activity of genes in the B compartment.[10]
Thermostability and Stress Responses
Thermoacidophiles have adapted to the extreme environments they inhabit by evolving specific mechanisms to deal with the high temperature and low pH environment. While these mechanisms aren’t necessarily unique to thermoacidophiles and can be found in thermophiles and acidophiles respectively, their incorporation into a single organism can lead to synergistic effects. For example, while DNA stability doesn’t necessarily depend solely on its
While some thermoacidophiles can grow optimally at a pH close to 0, their cytoplasm is often near neutral
While all of the previously mentioned adaptations that allow thermoacidophiles to survive harsh environment, it is inevitable that conditions in the cell reach a point where a protein is damaged by the extreme environment they inhabit. The thermosome is a common tool in thermoacidophilic archaea. It generally consists of multiple alpha and beta subunits, the proportions of which in the structure vary with temperature.[14] The structure of the thermosome resembles a capsule with openings at each end. Unlike bacterial thermosomes, which have a separate subunit that closes off the internal space, the archaeal thermosome undergoes a conformational change to close the openings at each end via the hydrolysis of ATP. The thermosome closes around the protein and offers a more stable environment for the protein to refold properly. If the thermosome cannot refold the denatured protein, the protein is tagged by ubiquitin for degradation by the proteasome.[14]