Free-Space Transmission Method for the Characterization of Dielectric and Magnetic Materials at Microwave Frequencies

Abstract

on the existence of chromatin in the latter, which results in a closed, less accessible genome (Struhl 1999). Despite the fact that this view may be too simplistic—there are struc-tural proteins associated with the DNA in prokaryotes (Anuchin et al. 2011) and eukaryotes use more repressors than anticipated (Kemmeren et al. 2014)—the general con-cept still stands true (Estrada et al. 2016). Strikingly, the human genome harbours the blueprint of about 200 highly specialized cell types characterized by very different mor-phology, metabolism and capacities including, for exam-ple, neurons, hepatocytes or gametes. By comparison, most prokaryotes have a limited range of cellular states. There-fore, the invention of chromatin may have been pivotal for the emergence of highly differentiated cell types, most likely because the expression of specific programmes must be tightly regulated to allow diverse and sometimes antago-nistic differentiated states to co-exist. For example, yeast differentiation during gametogenesis must be very strictly limited to diploid cells to avoid massive cell death resulting from haploid meiosis and recent data support that chroma-tin-based mechanisms play a key role in that process (van Werven et al. 2012). The understanding of how chromatin is established and how it contributes with most, if not all nuclear processes including transcription, DNA replication, DNA repair, recombination or chromosome segregation therefore constitutes an outstanding focus in current biol-ogy. In that context, an important and long-standing ques-tion is to decipher how nucleosomes, which constitute the basic unit of chromatin, are positioned genome-wide. There have been abundant debates about what dictates the posi-tion of nucleosomes with models fully relying on biophys-ics—the position is DNA encoded—and a model encom-passing a layer of active, ATP-dependent modelling of the chromatin template. The reader is redirected to excel-lent reviews addressing these issues (Korber 2012; Lieleg Abstract The occupancy of nucleosomes governs access to the eukaryotic genomes and results from a combination of biophysical features and the effect of ATP-dependent remodelling complexes. Most promoter regions show a conserved pattern characterized by a nucleosome-depleted region (NDR) flanked by nucleosomal arrays. The con-served RSC remodeler was reported to be critical to estab-lish NDR in vivo in budding yeast but other evidences sug-gested that this activity may not be conserved in fission yeast. By reanalysing and expanding previously published data, we propose that NDR formation requires, at least par-tially, RSC in both yeast species. We also discuss the most prominent biological role of RSC and the possibility that non-essential subunits do not define alternate versions of the complex.