The short-sequence design of DNA and its involvement in the 3-D structure of the genome.

Recent investigations have shown that isochores are characterized by a 3-D structure which is primarily responsible for the topology of chromatin domains. More precisely, an analysis of human chromosome 21 demonstrated that low-heterogeneity, GC-poor isochores are characterized by the presence of oligo-Adenines that are intrinsically stiff, curved and unfavorable for nucleosome binding. This leads to a structure of the corresponding chromatin domains, the Lamina Associated Domains, or LADs, which is well suited for interaction with the lamina. In contrast, the high-heterogeneity GC-rich isochores are in the form of compositional peaks and valleys characterized by increasing gradients of oligo-Guanines in the peaks and oligo-Adenines in the valleys that lead to increasing nucleosome depletions in the corresponding chromatin domains, the Topological Associating Domains, or TADs. These results encouraged us to investigate in detail the di- and tri-nucleotide profiles of 100 Kb segments of chromosome 21, as well as those of the di- to octa-Adenines and di- to octa-Guanines in some representative regions of the chromosome. The results obtained show that the 3-D structures of isochores and chromatin domains depend not only upon oligo-Adenines and oligo-Guanines but also, to a lower but definite extent, upon the majority of di- and tri-nucleotides. This conclusion has strong implications for the biological role of non-coding sequences.

Composition profile of the human genome at the chromosome level.

The compositional properties of the human genome have been extensively studied. These analyses focused mainly in isochores. With the availability of the human genome and several molecular techniques, new studies were performed, showing that nucleotide composition is related to three processes: gene expression, replication and recombination. Nevertheless, these studies usually focused on regions at the sub-chromosomal level. Here we study the compositional differences among chromosomes, considering structural and functional aspects using the chromosomes as the units of analysis. We show that: i) chromosomes are compositionally consistent units; ii) there exists a correlation between their GC content and size and location within the nucleus, and iii) the three processes mentioned above are linked to compositional properties at the chromosomal level. These results support the existence of a link between composition and spatial/structural/functional features of entire chromosomes. The Evolutionary mechanisms and forces underlying these patterns remain open questions.

Trends of amino acid usage in the proteins from the human genome.

Correspondence analysis of amino acid usage was applied to 14,815 complete proteins from the human genome. We found that three major factors influence the variability of amino acidic composition of these proteins, explaining, respectively 20.4%, 14.7%, and 9.9% of the total variability. The first trend is strongly correlated with the GC content of first and second codon positions and is also significantly correlated with the GC level of the corresponding flanking regions and introns. Therefore, the main force shaping amino acid usage among human proteins are the compositional constraints determined by the isochore in which each gene is embedded. The second trend correlates with the hydropathy of each protein and with the frequency of beta-strands. Finally, the third trend is strongly associated with the usage of Cys and the frequency of alpha-helices.

CpG islands are the second main factor shaping codon usage in human genes.

A correspondence analysis of codon usage in human genes revealed, as expected, that the first axis is strongly correlated with the base composition at synonymous third codon positions. At one extreme of the second axis were localized genes with a high frequency of NCG and CGN codons. The great majority of these sequences were embedded in CpG islands, while the opposite is true for the genes placed at the other extreme. The two main conclusions of this paper are: (1) the influence of CpG islands on codon usage, and (2) since the second axis is orthogonal (and therefore independent) of the first, GC3-rich genes are not necessarily associated with CpG islands.

Genomic GC content prediction in prokaryotes from a sample of genes.

GC level is a key feature in prokaryotic genomes. Widely employed in evolutionary studies, new insights appear however limited because of the relatively low number of characterized genomes. Since public databases mainly comprise several hundreds of prokaryotes with a low number of sequences per genome, a reliable prediction method based on available sequences may be useful for studies that need a trustworthy estimation of whole genomic GC. As the analysis of completely sequenced genomes shows a great variability in distributional shapes, it is of interest to compare different estimators. Our analysis shows that the mean of GC values of a random sample of genes is a reasonable estimator, based on simplicity of the calculation and overall performance. However, usually sequences come from a process that cannot be considered as random sampling. When we analyzed two introduced sources of bias (gene length and protein functional categories) we were able to detect an additional bias in the estimation for some cases, although the precision was not affected. We conclude that the mean genic GC level of a sample of 10 genes is a reliable estimator of genomic GC content, showing comparable accuracy with many widely employed experimental methods.