Archaeal histone-based chromatin structures regulate transcription elongation rates.

Many archaea encode and express histone proteins to compact their genomes. Archaeal and eukaryotic histones share a near-identical fold that permits DNA wrapping through select histone-DNA contacts to generate chromatin-structures that must be traversed by RNA polymerase (RNAP) to generate transcripts. As archaeal histones can spontaneously assemble with a single histone isoform, single-histone chromatin variants provide an idealized platform to detail the impacts of distinct histone-DNA contacts on transcription efficiencies and to detail the role of the conserved cleavage stimulatory factor, Transcription Factor S (TFS), in assisting RNAP through chromatin landscapes. We demonstrate that substitution of histone residues that modify histone-DNA contacts or the three-dimensional chromatin structure result in radically altered transcription elongation rates and pausing patterns. Chromatin-barriers slow and pause RNAP, providing regulatory potential. The modest impacts of TFS on elongation rates through chromatin landscapes is correlated with TFS-dispensability from the archaeon Thermococcus kodakarensis. Our results detail the importance of distinct chromatin structures for archaeal gene expression and provide a unique perspective on the evolution of, and regulatory strategies imposed by, eukaryotic chromatin.

Histones direct site-specific CRISPR spacer acquisition in model archaeon.

CRISPR-Cas systems provide heritable immunity against viruses and other mobile genetic elements by incorporating fragments of invader DNA into the host CRISPR array as spacers. Integration of new spacers is localized to the 5' end of the array, and in certain Gram-negative Bacteria this polarized localization is accomplished by the integration host factor. For most other Bacteria and Archaea, the mechanism for 5' end localization is unknown. Here we show that archaeal histones play a key role in directing integration of CRISPR spacers. In Pyrococcus furiosus, deletion of either histone A or B impairs integration. In vitro, purified histones are sufficient to direct integration to the 5' end of the CRISPR array. Archaeal histone tetramers and bacterial integration host factor induce similar U-turn bends in bound DNA. These findings indicate a co-evolution of CRISPR arrays with chromosomal DNA binding proteins and a widespread role for binding and bending of DNA to facilitate accurate spacer integration.

Archaeal transcription.

Increasingly sophisticated biochemical and genetic techniques are unraveling the regulatory factors and mechanisms that control gene expression in the Archaea. While some similarities in regulatory strategies are universal, archaeal-specific regulatory strategies are emerging to complement a complex patchwork of shared archaeal-bacterial and archaeal-eukaryotic regulatory mechanisms employed in the archaeal domain. The prokaryotic archaea encode core transcription components with homology to the eukaryotic transcription apparatus and also share a simplified eukaryotic-like initiation mechanism, but also deploy tactics common to bacterial systems to regulate promoter usage and influence elongation-termination decisions. We review the recently established complete archaeal transcription cycle, highlight recent findings of the archaeal transcription community and detail the expanding post-initiation regulation imposed on archaeal transcription.

FttA is a CPSF73 homologue that terminates transcription in Archaea.

Regulated gene expression is largely achieved by controlling the activities of essential, multisubunit RNA polymerase transcription elongation complexes (TECs). The extreme stability required of TECs to processively transcribe large genomic regions necessitates robust mechanisms to terminate transcription. Efficient transcription termination is particularly critical for gene-dense bacterial and archaeal genomes1-3 in which continued transcription would necessarily transcribe immediately adjacent genes and result in conflicts between the transcription and replication apparatuses4-6; the coupling of transcription and translation7,8 would permit the loading of ribosomes onto aberrant transcripts. Only select sequences or transcription termination factors can disrupt the otherwise extremely stable TEC and we demonstrate that one of the last universally conserved archaeal proteins with unknown biological function is the Factor that terminates transcription in Archaea (FttA). FttA resolves the dichotomy of a prokaryotic gene structure (operons and polarity) and eukaryotic molecular homology (general transcription apparatus) that is observed in Archaea. This missing link between prokaryotic and eukaryotic transcription regulation provides the most parsimonious link to the evolution of the processing activities involved in RNA 3'-end formation in Eukarya.