The availability of oxygen is a major environmental factor for many microbes, in particular for bacteria such as Shewanella species, which thrive in redox-stratified environments. One of the best-studied systems involved in mediating the response to changes in environmental oxygen levels is the Arc two-component system of Escherichia coli, consisting of the sensor kinase ArcB and the cognate response regulator ArcA. An ArcA ortholog was previously identified in Shewanella, and as in Escherichia coli, Shewanella ArcA is involved in regulating the response to shifts in oxygen levels. Here, we identified the hybrid sensor kinase SO_0577, now designated ArcS, as the previously elusive cognate sensor kinase of the Arc system in Shewanella oneidensis MR-1. Phenotypic mutant characterization, transcriptomic analysis, protein-protein interaction, and phosphotransfer studies revealed that the Shewanella Arc system consists of the sensor kinase ArcS, the single phosphotransfer domain protein HptA, and the response regulator ArcA. Phylogenetic analyses suggest that HptA might be a relict of ArcB. Conversely, ArcS is substantially different with respect to overall sequence homologies and domain organizations. Thus, we speculate that ArcS might have adopted the role of ArcB after a loss of the original sensor kinase, perhaps as a consequence of regulatory adaptation to a redox-stratified environment.

The thermoacidophilic crenarchaeote Sulfolobus acidocaldarius displays three distinct type IV pili-like structures on its surface: (i) the flagellum, (ii) the UV-induced pili and (iii) the adhesive pili. In bacteria, surface appendages play an important role in the spatial organization of cells from initial surface attachment to the development of mature community structures. To investigate the influence of the diverse set of type IV pili-like structures in S. acidocaldarius, single, double and triple mutants lacking the cell surface appendages were constructed and analysed for their behaviour in attachment assays and during biofilm formation. A heat stable green fluorescent protein was employed the first time in a hyperthermophilic archaeon. A codon adjusted eCGP123 was expressed to study mixed biofilms of different deletion mutants to understand the interplay of the surface structures during biofilm formation. During this process the deletion of the adhesive pili and UV-induced pili led to the most pronounced effects, either an increase in cell density or increased cluster formation respectively. However, all three cell surface appendages played a role in the colonization of surfaces and only the interplay of all three appendages leads to the observed wild-type biofilm phenotype.

Archaea display a variety of type IV pili on their surface and employ them in different physiological functions. In the crenarchaeon Sulfolobus acidocaldarius the most abundant surface structure is the aap pilus (archaeal adhesive pilus). The construction of in frame deletions of the aap genes revealed that all the five genes (aapA, aapX, aapE, aapF, aapB) are indispensible for assembly of the pilus and an impact on surface motility and biofilm formation was observed. Our analyses revealed that there exists a regulatory cross-talk between the expression of aap genes and archaella (formerly archaeal flagella) genes during different growth phases. The structure of the aap pilus is entirely different from the known bacterial type IV pili as well as other archaeal type IV pili. An aap pilus displayed 3 stranded helices where there is a rotation per subunit of ~ 138° and a rise per subunit of ~ 5.7 Å. The filaments have a diameter of ~ 110 Å and the resolution was judged to be ~ 9 Å. We concluded that small changes in sequence might be amplified by large changes in higher-order packing. Our finding of an extraordinary stability of aap-pili possibly represents an adaptation to harsh environments that S. acidocaldarius encounters.

Summary In response to a variety of environmental cues, prokaryotes can switch between a motile and a sessile, biofilm‐forming mode of growth. The regulatory mechanisms and signaling pathways underlying this switch are largely unknown in archaea but involve small winged helix‐turn‐helix DNA‐binding proteins of the archaea‐specific Lrs14 family. Here, we study the Lrs14 member AbfR1 of Sulfolobus acidocaldarius. Small‐angle X‐ray scattering data are presented, which are consistent with a model of dimeric AbfR1 in which dimerization occurs via an antiparallel coiled coil as suggested by homology modeling. Furthermore, solution structure data of AbfR1‐DNA complexes suggest that upon binding DNA, AbfR1 induces deformations in the DNA. The wing residues tyrosine 84 and serine 87, which are phosphorylated in vivo, are crucial to establish stable protein‐DNA contacts and their substitution with a negatively charged glutamate or aspartate residue inhibits formation of a nucleoprotein complex. Furthermore, mutation abrogates the cellular abundance and transcription regulatory function of AbfR1 and thus affects the resulting biofilm and motility phenotype of S. acidocaldarius. This work establishes a novel wHTH DNA‐binding mode for Lrs14‐like proteins and hints at an important role for protein phosphorylation as a signal transduction mechanism for the control of biofilm formation and motility in archaea.

In Archaea, type IV prepilins and prearchaellins are processed by designated signal peptidase III (SPaseIII) prior to their incorporation into pili and the archaellum, respectively. These peptidases belong to the family of integral membrane aspartic acid proteases that contain two essential aspartate residues of which the second aspartate is located in a conserved GxGD motif. To this group also bacterial type IV prepilin peptidases, Alzheimer disease-related secretases, signal peptide peptidases and signal peptide peptidase-like proteases in humans belong. Here we have performed detailed in vivo analyses to understand the cleavage activity of PibD, SPaseIII from the thermoacidophilic crenarchaeon Sulfolobus acidocaldarius. Using an already established in vivo heterologous system cleavage assay, we could successfully identify the key amino acid residues essential for catalysis of PibD. Furthermore, in trans complementation of a pibD S. acidocaldarius deletion mutant with PibD variants having substituted key amino acids has consolidated our observations of the importance of these residues in catalysis. Based on our data, we propose to re-define class III peptidases/type IV prepilin/prearchaellin peptidases as GxHyD group (rather than GxGD) of proteases [Hy-hydrophobic amino acid].