Periodicities of 10–11 bp as Indicators of the Supercoiled State of Genomic DNA

DNA sequences contain information about the bendability and native conformation of DNA. For example, a repetition of certain dinucleotides at distances of 10–11 bp supports wrapping around nucleosomes and supercoiled structures of bacterial DNA. We analyzed 86 eubacterial genomes, 16 archaea, and six genomes of higher eukaryotes. First, we discuss whether or not the observed periodicities represent indeed bendability signals. This claim is confirmed since: (1) dinucleotide signals are of comparable size to mononucleotide signals, (2) the signals are present in non-coding DNA as well, and (3) repeat masking has only a minor effect on 10–11 bp periodicities. Moreover, the periodicities persist up to 150 bp, comparable to the nucleosome size. We show that doublet peaks in Caenorhabditis elegans and some prokaryotes can be traced back to long-ranging modulations. In mammalian genomes, we find consistently spectral peaks as observed earlier in human chromosomes 20, 21 and 22. It has been shown in previous studies that archaea have periods of 10 bp, whereas eubacteria exhibit 11 bp periodicities. These differences reflect different supercoiled states of microbial DNA. Is the period of 10 bp an archaeal or a thermophilic feature? This question is addressed by relating periodicities to optimal growth temperatures. It turns out that the archaea Methanopyrus kandleri (topt=80 °C) and a Halobacterium strain (topt=42 °C) both have longer periods of about 11 bp. Eubacterial genomes have consistently periods around 11 bp indicative of negative supercoiling.