# Sulfolobus solfataricus

Saccharolobus solfataricus is a species of thermophilic archaeon. It was transferred from the genus Sulfolobus to the new genus Saccharolobus with the description of Saccharolobus caldissimus in 2018.
It was first discovered and isolated from the Solfatara volcano (Pisciarelli-Campania, Italy) in 1980 by two German microbiologists Karl Setter and Wolfram Zillig.
However, these organisms are not isolated to volcanoes but are found all over the world in places such as hot springs. The species grows best in temperatures around 80 °C, a pH level between 2 and 4, and with enough sulfur for S. solfataricus to metabolize in order to gain energy. These conditions qualify it as an extremophile and it is specifically known as a thermoacidophile because of its preference for high temperatures and low pH levels. It is also aerobic and heterotropic due to its metabolic system. Being an autotroph, it receives energy by growing on sulfur or even a variety of organic compounds. It usually has a spherical cell shape and it makes frequent lobes.
Currently, it is the most widely studied organism within the Thermoproteota branch. Solfataricus are examined for their methods of DNA replication, cell cycle, chromosomal integration, transcription, RNA processing, and translation. All of the data points to the organism having a large percent of archaeal-specific genes, which shows the differences between the three types of microbes: archaea, bacteria, and eukaryote.

## Genome

Sulfolobus solfataricus is the most studied microorganism from a molecular, genetic, and biochemical point of view for its ability to thrive in extreme environments. It can grow easily in the laboratory; moreover, it can exchange genetic material through processes of transformation, transduction. and conjugation.
The major motivation for sequencing these microorganisms is the thermostability of proteins that normally denature at a high temperature. The complete sequence the genome of S. solfataricus was completed in 2001. On a single chromosome, there are 2,992,245 base pairs which encode for 2,977 proteins and copious RNAs. One-third of S. solfataricus encoded proteins have no homologs in other genomes. For the remaining encoded proteins, 40% are specific to Archaea, 12% are shared with Bacteria, and 2.3% are shared with Eukaryote; 33% of these proteins are encoded exclusively in Sulfolobus. A high number of open reading frames (ORFs) are highly similar in Thermoplasma.
Small nucleolar RNAs (snoRNAs), already present in eukaryotes, have also been identified in S. solfataricus and S. acidolcaldarius. They are already known for the role they play in post-transcriptional modifications and removal of introns from ribosomal RNA in Eukaryote.
The genome of Sulfolobus is characterized by the presence of short tandem repeats, insertion and repetitive elements. It has a wide range of diversity with 200 different insertion sequence elements.

### Thermophilic reverse gyrase

The stabilization of the double helix against denaturation, in the Archaea, is due to the presence of a particular thermophilic enzyme, reverse gyrase. It was discovered in hyper-thermophilic and thermophilic Archaea and Bacteria. There are two genes in Sulfolobus that each encode a reverse gyrase. It is defined as an atypical DNA topoisomerase and the basic activity consists of the production of positive supercoils in a closed circular DNA. Positive supercoiling is important to prevent the formation of open complexes. Reverse gyrases are composed of two domains: the first one is the helicase and second one is the topoisomerase I. A possible role of reverse gyrase could be the use of positive supercoiling to assemble chromatin-like structures. In 1997, scientists discovered another important feature of Sulfolobus: a type-II topoisomerase, called TopoVI, whose A subunit is homologous to the meiotic recombination factor, Spo11, which plays a predominant role in the initiation of meiotic recombination in all Eukaryotes.
S. solfataricus is composed of three topoisomerases of type I, TopA and two reverse gyrases, TopR1 and TopR2, and one topoisomerase of type II, TopoVI.

### DNA binding proteins

In the phylum Thermoproteota, there are three proteins that bind to the minor groove of DNA-like histones, Alba, Cren7, and Sso7d, that are modified after the translation process. These histones are small and have been found in several strains of Sulfolobus but not in other genomes. Chromatin protein in Sulfolobus represent 1-5% of the total genome. They can have both structural and regulatory functions. These look like human HMG-box proteins, because of their influence on genomes, expression and stability, and epigenetic processes. In species lacking histones, they can be acetylated and methylated like eukaryotic histones. Sulfolobus strains present different peculiar DNA binding proteins, such as the Sso7d protein family. They stabilize the double helix, preventing denaturation at high temperature and thus promoting annealing above the melting point.
The major component of Archaea chromatin is represented by Sac10b family protein known as Alba (acetylation lowers binding affinity). These proteins are small, basic, and dimeric nucleic acid-binding proteins. Furthermore, it is conserved in most sequenced Archaea genomes. The acetylation state of Alba affects promoter access and transcription in vitro, whereas the methylation state of another Sulfolobus chromatin protein, Sso7D, is altered by culture temperature.
The work of Wolfram Zillig's group, representing early evidence of the eukaryotic characteristics of transcription in Archaea, has since made Sulfolobus an ideal model system for transcription studies. Recent studies in Sulfolobus, in addition to other Archaea species, mainly focus on the composition, function, and regulation of the transcription machinery and on fundamental conserved aspects of this process in both Eukaryotes and Archaea.

## DNA transfer

Exposure of Saccharolobus solfataricus to the DNA damaging agents, ultraviolet (UV) irradiation, bleomycin, or mitomycin C, induces cellular aggregation. Other physical stressors, such as changes in pH or temperature shift, do not induce aggregation, suggesting that the induction of aggregation is caused specifically by DNA damage. Ajon et al. showed that UV-induced cellular aggregation mediates chromosomal marker exchange with high frequency. Recombination rates exceeded those of uninduced cultures by up to three orders of magnitude. Frols et al. and Ajon et al. hypothesized that the UV-induced DNA transfer process and subsequent homologous recombinational repair represents an important mechanism to maintain chromosome integrity. This response may be a primitive form of sexual interaction, similar to the more well-studied bacterial transformation that is also associated with DNA transfer between cells, leading to homologous recombinational repair of DNA damage.

## Metabolism

Sulfolobus solfataricus is known to grow by chemoorganotrophy, in the presence of oxygen, on a variety of organic compounds such as sugars, alcohols, amino acids, and aromatic compounds like phenol.
It uses a modified Entner-Doudroff pathway for glucose oxidation and the resulting pyruvate molecules can be totally mineralized in a TCA cycle.
Molecular oxygen is the only known electron acceptor at the end of the electron transport chain. Other than organic molecules, this Archaea species can also utilize hydrogen sulfide and elementary sulfur as electron donors and fix CO$_2$, possibly by means of the HP/HB cycle, making it also capable of living by chemoautotrophy. Recent studies have also found the capability of growing, albeit slowly, by oxidizing molecular hydrogen.

### Ferredoxin

Ferredoxin is suspected to act as the major metabolic electron carrier in S. solfataricus. This contrasts with most species within the Bacteria and Eukaryote groups of organisms, which generally rely on nicotinamide adenine dinucleotide hydrogen (NADH) as the main electron carrier. S. solfataricus has strong eukaryotic features coupled with many uniquely archaeal-specific abilities. The results of the findings came from the varied methods of their DNA mechanisms, cell cycles, and transitional apparatus. Overall, the study was a prime example of the differences found in Thermoproteota and "Euryarchaeota".

## Ecology
### Habitat

S. solfataricus is an extreme thermophile Archaea, as the rest of the species of the genus Sulfolobus, has optimal growth conditions in strong volcanic activity areas, with high temperatures and very acidic pH. These specific conditions are typical of volcanic areas such as geyser or thermal springs. In fact, the most studied countries where these microorganisms were found are U.S.A. (Yellowstone National Park), New Zealand, Island and Italy, notoriously famous for volcanic phenomena. A study conducted by a team of Indonesian scientists has also shown the presence of a Sulfolobus community in West Java, confirming that high temperatures, low pH, and the presence of sulfur are necessary conditions for the growth of these microbes.

### Soil acidification

S. solfataricus is able to oxidize sulfur according to metabolic strategy. One of the products of these reactions is H+ and, consequentially, it results in a slowly acidification of the surrounding area. Soil acidification increases in places where there are emissions of pollutants from industrial activity, and this process reduces the number of heterotrophic bacteria involved in decomposition, which are fundamental to recycling organic matter and ultimately to fertilizing soil.

## Biotechnology: Untapping the resource Sulfolobus

Today, in many fields of application, there is interest in using S. sulfataricus as a source of thermal stability enzymes for research and diagnostics as well as in the food, textile, cleaning, and pulp and paper industries. Furthermore, this enzyme is overloaded due to its catalytic diversity, high pH, and temperature stability, increased to organic solvents and resistance to proteolysis.
At present, tetra ester lipids, membrane vesicles with antimicrobial properties, trehalose components, and new β-galactooligosaccharides are becoming increasingly important.

### β-galactosidase

The thermostable enzyme β-galactosidase was isolated from the extreme thermophile archaebacterial S. solfataricus, strain MT-4.
This enzyme is utilized in many industrial processes of lactose containing fluids by purifying and characterizing their physicochemical properties.

### Proteases

The industry are interested in stable proteases as well as in many different Sulfolobus proteases that have been studied.
An active aminopeptidase associated with the chaperonin of S. solfataricus MT4 was described.
Sommaruga et al. (2014) also improved the stability and reaction yield of a well-characterized carboxypeptidase from S. solfataricus MT4 by magnetic nanoparticles immobilizing the enzyme.

### Esterases/Lipases

A new thermostable extracellular lipolytic enzyme serine arylesterase was originally discovered for their large action in the hydrolysis of organophosphates from the thermoacidophilic archaeon S. solfataricus P1.

### Chaperonins

In reaction to temperature shock (50.4 °C) in E. coli cells, a tiny warm stun protein (S.so-HSP20) from S.solfataricus P2 has been effectively used to improve tolerance to temperature.
Since chaperonin Ssocpn (920 kDa), which includes adenosine triphosphate (ATP), K+, and Mg$^2$ +, has not produced any additional proteins in S. solfataricus to supply collapsed and dynamic proteins from denatured materials, it was stored on an ultrafiltration cell, while the renatured substrates were moving through the film.

### Liposomes

Because of its tetraether lipid material, the membrane of extreme thermophilic Archaea is unique in its composition. Archaea lipids are a promising source of liposomes with exceptional stability of temperature, pH, and tightness against the leakage of solute. Such archaeosomes are possible instruments for the delivery of medicines, vaccines, and genes.