| dc.description.abstract | The domain of life known as Archaea, encompasses an astonishing array of
intricately diverse species thriving across a wide spectrum of highly varied
environments around the globe.
First discovered in the 1970s (recognized as archaeabacteria at the time), the
uncovering of a collection of methanogenic species initiated the development of the
kingdom now known as archaea. From this initial identification of anaerobic species
found in thermal springs 1,2 , the search for other extremophile species then began.
The subsequent discovery of halophilic species from salt lakes and salt-saturated
soils 3 , as well as thermophilic species within hot acidic sulphur pools 4 , led to an
array of novel environmentally distinct organisms. Comparisons of these habitually
diverse species found both biochemical similarities as well as significant homology
between their 16S rRNA and tRNA molecules 5,6 . This ultimately led to the discovery
of a defined branch of organisms, distinct from bacteria, that would later be classified
as the archaeal kingdom.
Whilst archaeal species have conquered some of the most extreme ecological
niches on planet earth 7–10 , many archaea have been located within a plethora of
ambient environments also 11–14 , contributing to nutrient cycles and performing
organic decompositions within mesophilic life systems. This vast success across the
archaeal kingdom is due to a list of intricate developments. Genetic flexibility has
allowed for key adaptations, promoting success over a vast array of niches. The
generation of specialised enzymes, optimally functional across temperatures outside the range of prokaryotic and eukaryotic counterparts, has allowed for growth in many
areas unchallenged by other life forms. The utilisation of huge versatile lists of
metabolites has also seen archaea populate areas of the globe that no other
kingdom of life can. However, one other such key area has also played a pinnacle
role, the archaeal surface structures.
Within the course of evolution, prokaryotic and thus by association archaeal
organisms have developed a broad spectrum of cell enveloping surface structures.
The most ubiquitous of these are surface layers (S-layers), which form a
proteinaceous lattice completely covering the cell. S-layers are present in almost all
archaea, and shared amongst various species of almost every taxonomic group of
walled bacteria. These paracrystalline structures provide organisms with selection
advantages through provision of a variety of novel functions: acting as protective
coats to survive harsh conditions, molecular sieves to select for macromolecule
access into the cytoplasm, environments for ion traps to localise metals for
functionality, virulence protection, cell shape and division control measures 15–18 .
In association with the S-layer, archaea also produce a variety of filamentous
appendages that protrude through it. Providing a number of distinct biological
functions such as adhesion, biofilm formation, motility, species-species interaction,
as well as the exchange of genetic material between cells 19,20 .
The aims within this PhD thesis were firstly to obtain a deeper understanding of the
arsenal of filamentous appendages within a single specific archaeal model organism,
Sulfolobus acidocaldarius. This was achieved by employing Electron Cryogenic
Microscopy (CryoEM), which then revealed the structures of three distinct filaments –
archaella, archaeal-adhesive pili (Aap) and threads. Leveraging bioinformatics, these
three structures and by association, their machinery complexes were then analysed
in correlation with each other and in relation to the archaeal S-layer. Thereby offering
valuable insights into their evolutionary implications. Fundamentally, this data now
provides insight in to how these structures link to other species, primarily within the
archaeal world, but additionally how they relate to bacterial filaments.
In addition, two evolutionary distinct and S-layer-less species of the phylum
Thermoplasmatales, Cunicuplasma divulgatum and Oxyplasma meridanum were
investigated. The subsequent structural findings shed new light on the question
whether archaea lacking S-layers produce canonical or divergent archaella and pili.
An emphasis was placed on the filament operons of these species, providing both
genetic and structural insights into how archaeal filament machinery differs between
filament types, as well as between species of Archaea from different Phyla.
The overarching objective of this thesis was to contribute a piece to our ongoing
efforts in unravelling the mysteries of archaeal life. | en_GB |
