Organization and Regulation of Chromatin by Liquid-Liquid Phase Separation, bioRxiv, 2019-01-18

Genomic DNA is highly compacted in the nucleus of eukaryotic cells as a nucleoprotein assembly called chromatin. The basic unit of chromatin is the nucleosome, where ~146 base pair increments of the genome are wrapped and compacted around the core histone proteins. Further genomic organization and compaction occur through higher order assembly of nucleosomes. This organization regulates many nuclear processes, and is controlled in part by histone post-transtranslational modifications and chromatin-binding proteins. Mechanisms that regulate the assembly and compaction of the genome remain unclear. Here we show that in the presence of physiologic concentrations of mono- and divalent salts, histone tail-driven interactions drive liquid-liquid phase separation (LLPS) of nucleosome arrays, resulting in substantial condensation. Phase separation of nucleosomal arrays is inhibited by histone acetylation, whereas histone H1 promotes phase separation, further compaction, and decreased dynamics within droplets, mirroring the relationship between these modulators and the accessibility of the genome in cells. These results indicate that under physiologically relevant conditions, LLPS is an intrinsic behavior of the chromatin polymer, and suggest a model in which the condensed phase reflects a genomic 'ground state' that can produce chromatin organization and compaction in vivo. The dynamic nature of this state could enable known modulators of chromatin structure, such as post-translational modifications and chromatin binding proteins, to act upon it and consequently control nuclear processes such as transcription and DNA repair. Our data suggest an important role for LLPS of chromatin in the organization of the eukaryotic genome.

biorxiv biophysics 100-200-users 2019

Recording of sonic attacks on U.S. diplomats in Cuba spectrally matches the echoing call of a Caribbean cricket, bioRxiv, 2019-01-04

Beginning in late 2016, diplomats posted to the United States embassy in Cuba began to experience unexplained health problems including ear pain, tinnitus, vertigo, and cognitive difficulties which reportedly began after they heard strange noises in their homes or hotel rooms. In response, the U.S. government dramatically reduced the number of diplomats posted at the U.S. embassy in Havana. U.S. officials initially believed a sonic attack might be responsible for their ailments. The sound linked to these attacks, which has been described as a high-pitched beam of sound, was recorded by U.S. personnel in Cuba and released by the Associated Press (AP). Because these recordings are the only available non-medical evidence of the sonic attacks, much attention has focused on identifying health problems and the origin of the acoustic signal. As shown here, the calling song of the Indies short-tailed cricket (Anurogryllus celerinictus) matches, in nuanced detail, the AP recording in duration, pulse repetition rate, power spectrum, pulse rate stability, and oscillations per pulse. The AP recording also exhibits frequency decay in individual pulses, a distinct acoustic signature of cricket sound production. While the temporal pulse structure in the recording is unlike any natural insect source, when the cricket call is played on a loudspeaker and recorded indoors, the interaction of reflected sound pulses yields a sound virtually indistinguishable from the AP sample. This provides strong evidence that an echoing cricket call, rather than a sonic attack or other technological device, is responsible for the sound in the released recording. Although the causes of the health problems reported by embassy personnel are beyond the scope of this paper, our findings highlight the need for more rigorous research into the source of these ailments, including the potential psychogenic effects, as well as possible physiological explanations unrelated to sonic attacks.

biorxiv biophysics 200-500-users 2019

 

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