Segmented structure of separate and transposable DNA and RNA elements as suggested by their size distributions

Wavelet Analysis of DNA Bending Profiles reveals Structural Constraints on the Evolution of Genomic Sequences

Analyses of genomic DNA sequences have shown in previous works that base pairs are correlated at large distances with scale-invariant statistical properties. We show in the present study that these correlations between nucleotides (letters) result in fact from long-range correlations (LRC) between sequence-dependent DNA structural elements (words) involved in the packaging of DNA in chromatin. Using the wavelet transform technique, we perform a comparative analysis of the DNA text and of the corresponding bending profiles generated with curvature tables based on nucleosome positioning data. This exploration through the optics of the so-called `wavelet transform microscope' reveals a characteristic scale of 100-200 bp that separates two regimes of different LRC. We focus here on the existence of LRC in the small-scale regime (≲ 200 bp). Analysis of genomes in the three kingdoms reveals that this regime is specifically associated to the presence of nucleosomes. Indeed, small scale LRC are observed in eukaryotic genomes and to a less extent in archaeal genomes, in contrast with their absence in eubacterial genomes. Similarly, this regime is observed in eukaryotic but not in bacterial viral DNA genomes. There is one exception for genomes of Poxviruses, the only animal DNA viruses that do not replicate in the cell nucleus and do not present small scale LRC. Furthermore, no small scale LRC are detected in the genomes of all examined RNA viruses, with one exception in the case of retroviruses. Altogether, these results strongly suggest that small-scale LRC are a signature of the nucleosomal structure. Finally, we discuss possible interpretations of these small-scale LRC in terms of the mechanisms that govern the positioning, the stability and the dynamics of the nucleosomes along the DNA chain. This paper is maily devoted to a pedagogical presentation of the theoretical concepts and physical methods which are well suited to perform a statistical analysis of genomic sequences. We review the results obtained with the so-called wavelet-based multifractal analysis when investigating the DNA sequences of various organisms in the three kingdoms. Some of these results have been announced in B. Audit et al. [1, 2].

Distribution of rare triplets along mRNA and their relation to protein folding

It is believed that pausing during mRNA translation plays some role in ensuring proper folding of newly synthesized sections of a protein chain. Such pausing occurs when rare triplets are encountered in the mRNA, as it takes additional time for the corresponding rare species of tRNA to be delivered. To determine whether pause sites are non-randomly distributed along prokaryotic mRNA (cDNA), we have located clusters of rare triplets in cDNA sequences from 21 different bacteria. From the individual profiles of local codon frequencies calculated with various windows, the positions of the clusters of the rarest codons were taken for generation of the combined histograms of positional preferences of the pause sites. The histograms show that in the prokaryotic sequences, the pause sites are located preferentially at the start positions and at about 155 triplets from the starts. To verify the generality of these observations, the data are grouped in six independent sets about 500 sequences each, all revealing the same features. A less prominent maximum is also seen at the triplet position 75. Judging by the amplitude of the peak at 155 triplets, an optimal cluster size is estimated to equal 18 triplets. The distance 155 closely corresponds to the sizes of typical protein folds and to earlier estimated prokaryotic protein sequence segments. This supports the suggestion of a role for translation pausing in the cotranslational folding of protein domains. The profiles of rare codons in mRNA can serve in the detection or prediction of boundaries between protein domains.

Escherichia coli genome is composed of two distinct types of nucleotide sequences

We calculated correlations of the nucleotide distributions along the E. coli genome. Subsequent cluster analysis of the correlation distributions showed that the genome was composed of two qualitatively different types of nucleotide sequences. The first type exhibited strong correlations of the genomic distributions of A with T and G with C, and high anticorrelations of A with C and G with T. In contrast, the second type was characterized by weak or negligible correlations typical of randomized sequences. Both types of sequences were almost equally abundant in the E. coli genome and their length varied from several hundred nucleotides to about 70 kilobases. They were not disjunct with respect to their (G + C) content but the high correlations and anticorrelations were rather characteristic for (A + T)-rich genomic segments. We offer possible explanations of the mosaic structure of the E. coli genome.

Variations of the mononucleotide and short oligonucleotide distributions in the genomes of various organisms

We calculated the variation coefficients of the mononucleotide and short oligonucleotide distributions in over 1700 long genomic sequences originating from six organisms to demonstrate that the human and Escherichia coli genomic sequences were the least and the most uniform, respectively. The most non-random genomic distributions were exhibited by the four canonical nucleotides, followed by the strong and weak nucleotides, while the distributions of purine or pyrimidine nucleotides and especially the distributions of (A+C) and (G+T) were significantly more uniform even in the human genome. In the human and mouse genomes, the highest coefficients of variation were further observed with the oligonucleotides where CG was combined with the strong nucleotides while its combination with the weak nucleotides significantly decreased the variation which, however, was still very high. High variation was also exhibited by the remaining oligonucleotides composed exclusively of the strong nucleotides or those containing only weak nucleotides. On the other hand, the distributions of oligonucleotides containing similar and especially the same numbers of the strong and weak nucleotides, but no CG or TA dinucleotide, were the most uniform. The information following from the present analysis will be useful not only in the identification of important genomic regions but also in computer simulations of the genomic nucleotide sequences in order to trace and reproduce the pathways of genome evolution.

Correlations and anticorrelations among nucleotide distributions along the genomes of various organisms

We have analyzed correlations of nucleotide distributions along more than 50 megabases of the longest sequenced parts of the human, mouse, Drosophila, Arabidopsis, yeast, E.coli and three kinds of viral genomes. The strongest correlations were observed between the distributions of C and G, in particular in the genome of Drosophila. This correlation was much weaker, though still strong, in the human genome and E.coli that exhibited the same level of this correlation. The C/G correlation hardly originates from the isochores because the isochores were not reported to occur in the genomes of Drosophila and E. coil. The genomic distribution curves of adenine and thymine were also positively correlated in all analyzed organisms except for the yeast where they were anticorrelated. Still stronger anticorrelations were, however, observed between the genomic distributions of A and C and between G and T. These genomic distributions anticorrelated almost generally and very strong. These anticorrelations are likely to originate from point mutations resulting from unrepaired GA mispairing as a replication intermediate. The C/A or G/T anticorrelation or compensation is a very strong and general new phenomenon that shapes the genomic nucleotide sequences.

Elucidating sequence codes: three codes for evolution

The sequences are related to evolution in several ways. First, they carry traces of a distant past. Two sequence features point to the earliest sequence organization. The universal hidden GCU-periodical pattern in mRNA suggests the earliest codons: GCU and its nine-point-change derivatives. They code for seven amino acids that by several criteria are also the oldest. Together it makes the earliest form of the triplet code, still recognizable in the extant sequences. Another feature present in the sequences, apparently, since separation of prokaryotes and eukaryotes, is hidden genome segmentation. Both protein-coding and noncoding sequences appear to have been formed by fusion of standard size units, about 360 bp (120 aa) in eukaryotes and 450 bp (150 aa) in prokaryotes. Presumably, the units have been functioning at some stage of evolution as autonomous single-gene size elements. There are sequence designs that promote evolution. One such design suitable for fast adaptation is the tandem repetition of identical sequences, so that their copy numbers in the repeat arrays would modulate (tune) the expression of nearby genes. The tandem repeat expansion diseases illustrate this mechanism in a dramatic way: overtuning of the respective gene expression leads to the disease.