The accessibility of the DNA is a primary determinant of gene expression, and is therefore of great interest to molecular geneticists. A central question is how nucleosomes are distributed around the regions at which transcription starts. The selection of the start site, or gene promoter, is the first crucial step in the conversion of genetic information into the bricks and mortar of all cells, the proteins. It turns out that these promoter sites are marked by the presence of a nucleosome-free zone flanked by a specific pattern of nucleosomes. The biological function of these gaps seems to be to provide accessible docking sites for the transcriptional machinery, which comprises a multiprotein complex consisting of many subunits.
Together with his PhD student Wolfram Moebius, Gerland has asked whether a simple physical principle might not account for the characteristic distribution of nucleosomes in the vicinity of transcription start sites. The researchers made use of the so-called Tonks model, which applies to interactions between diffusing particles that are confined to one dimension. "Provided one knows the position of a single particle, one can use the model to predict, in a statistical sense, the positions of the particles in the vicinity" ", says Wolfram Moebius, who is the first author on the new study. "In addition, one observes a typical pattern of oscillations in the particle density." The analyses showed that the model of a Tonks gas indeed describes the distribution of the nucleosomes with surprising precision.
"When we plug average values derived from a large set of promoter regions into the model, the calculations reproduce the typical range of variation in nucleosome density that we see in biological systems", explains Gerland. The new model agrees best with the bio
|Contact: Luise Dirscherl|