Employing an inducible gene in a hyper-negatively supercoiled E. coli strain they demonstrated that negative supercoiling increased ssDNA patch density compared to wild type and promoted a higher mutation rate. It will be interesting to know whether a similar effect is observed in eukaryotic

cells where the DNA is packaged into chromatin and levels of supercoiling are probably buffered. In eukaryotic cells the effects of supercoiling have to be considered in the context of chromatin but unfortunately, we know very little about this situation. At the level of the ‘twin supercoil domain’ the scenario seems simplistic; positive supercoiling ahead of the polymerase will destabilize nucleosome structure and negative supercoiling behind will promote reassembly [36], actions that seem entirely consistent with the thermodynamic demands of transcription through a chromatin fibre. However, the many check details models that purport to explain the mechanics of how polymerase does in fact transcribe through a nucleosome reflects our ignorance of the details [37]. Things are no clearer at higher levels of chromatin structure. The idea that supercoiling might be generated at one site, say at a transcriptionally active gene, and then transmitted through the chromatin fibre to another location to create or remodel a domain or to influence a distant process, hinges on the concept that torsion can be transmitted along the fibre

(Figure 4). Although we raised this issue, twenty-five years ago [38], the question essentially click here remains unanswered as the difficulty is multifaceted. We do not have a good understanding of Saracatinib mouse the structure(s) that the higher-order chromatin fibre adopts, and yet this will undoubtedly constitute a profound influence upon the ability to transmit supercoiling. In addition,

the composition and modification of the components of the fibre are also likely to affect its plasticity. Nucleosomes containing yeast histones are more sensitive to thermally induced torsional stress [39] than nucleosomes containing higher eukaryotic core histones suggesting, perhaps, a greater propensity for yeast chromatin to absorb rather than transmit negative supercoiling. In spite of these reservations pioneering single-molecule studies have attempted to provide an insight into this fundamental question. Using magnetic tweezers to introduce torsional stress into model chromatin fibres Bancaud et al. [ 40] found chromatin to be highly accommodating of supercoiling. To illustrate, they argued that supercoiling generated by transcribing 100 bp of DNA could be absorbed within a 10 kb chromatin fragment thereby diminishing the need for topoisomerase relaxation. Although such plasticity may not be typical of more condensed, native chromatin fibres, it does provide insight into the buffering capacity of chromatin to supercoiling and its transmission.