Ecophysiological significance of scale-dependent patterns in prokaryotic genomes unveiled by a combination of statistic and genometric analyses

Recent advances in HCV entry

ABSTRACT: 

The elucidation of the mechanisms by which HCV infects hepatocytes and replicates has been paramount for identifying therapeutic targets and developing the highly efficacious antiviral drugs from which we benefit today. The earliest stage of HCV infection is viral entry, a process in which a complex interplay is thought to occur between host molecules (including glycosaminoglycans, low-density lipoprotein receptor, CD81, SR-B1, CLDN1, OCLN, EGF receptor, ephrin type A receptor 2 and transferrin receptor 1) and envelope viral glycoproteins E1 and E2. The wealth of experimental data produced in the field of HCV entry is summarized in a proposed mechanism, updated to include the most recently published data on the topic. Compounds with putative entry-blocking and/or entry-inhibiting activity in vitro and in vivo are also briefly reviewed.

Exploration of community traits as ecological markers in microbial metagenomes

The rate of information collection generated by metagenomics is uncoupled with its meaningful ecological interpretation. New analytical approaches based on functional trait-based ecology may help to bridge this gap and extend the trait approach to the community level in vast and complex environmental genetic data sets. Here, we explored a set of community traits that range from nucleotidic to genomic properties in 53 metagenomic aquatic samples from the Global Ocean Sampling (GOS) expedition. We found significant differences between the community profile derived from the commonly used 16S rRNA gene and from the functional trait set. The traits proved to be valuable ecological markers by discriminating between marine ecosystems (coastal vs. open ocean) and between oceans (Atlantic vs. Indian vs. Pacific). Intertrait relationships were also assessed, and we propose some that could be further used as habitat descriptors or indicators of artefacts during sample processing. Overall, the approach presented here may help to interpret metagenomics data to gain a full understanding of microbial community patterns in a rigorous ecological framework.

A close relationship between primary nucleotides sequence structure and the composition of functional genes in the genome of prokaryotes

Comparative genomics is an essential tool to unravel how genomes change over evolutionary time and to gain clues on the links between functional genomics and evolution. In prokaryotes, the large, good quality, genome sequences available in public databases and the recently developed large-scale computational methods, offer an unprecedent view on the ecology and evolution of microorganisms through comparative genomics. In this work, we examined the links among genome structure (i.e., the sequential distribution of nucleotides itself by detrended fluctuation analysis, DFA) and genomic diversity (i.e., gene functionality by Clusters of Orthologous Genes, COGs) in 828 full sequenced prokaryotic genomes from 548 different bacteria and archaea species. DFA scaling exponent α indicated persistent long-range correlations (fractality) in each genome analyzed. Higher resolution power was found when considering the sequential succession of purine (AG) vs. pyrimidine (CT) bases than either keto (GT) to amino (AC) forms or strongly (GC) vs. weakly (AT) bonded nucleotides. Interestingly, the phyla Aquificae, Fusobacteria, Dictyoglomi, Nitrospirae, and Thermotogae were closer to archaea than to their bacterial counterparts. A strong significant correlation was found between scaling exponent α and COGs distribution, and we consistently observed that the larger α the more heterogeneous was the gene distribution within each functional category, suggesting a close relationship between primary nucleotides sequence structure and functional genes composition.

Comparative analysis of acidobacterial genomic fragments from terrestrial and aquatic metagenomic libraries, with emphasis on acidobacteria subdivision 6

The bacterial phylum Acidobacteria has a widespread distribution and is one of the most common and diverse phyla in soil habitats. However, members of this phylum have often been recalcitrant to cultivation methods, hampering the study of this presumably important bacterial group. In this study, we used a cultivation-independent metagenomic approach to recover genomic information from soilborne members of this phylum. A soil metagenomic fosmid library was screened by PCR targeting acidobacterial 16S rRNA genes, facilitating the recovery of 17 positive clones. Recovered inserts appeared to originate from a range of Acidobacteria subdivisions, with dominance of subdivision 6 (10 clones). Upon full-length insert sequencing, gene annotation identified a total of 350 open reading frames (ORFs), representing a broad range of functions. Remarkably, six inserts from subdivision 6 contained a region of gene synteny, containing genes involved in purine de novo biosynthesis and encoding tRNA synthetase and conserved hypothetical proteins. Similar genomic regions had previously been observed in several environmental clones recovered from soil and marine sediments, facilitating comparisons with respect to gene organization and evolution. Comparative analyses revealed a general dichotomy between marine and terrestrial genes in both phylogeny and G+C content. Although the significance of this homologous gene cluster across subdivision 6 members is not known, it appears to be a common feature within a large percentage of all acidobacterial genomic fragments recovered from both of these environments.

Examination of genome homogeneity in prokaryotes using genomic signatures

BACKGROUND

DNA word frequencies, normalized for genomic AT content, are remarkably stable within prokaryotic genomes and are therefore said to reflect a "genomic signature." The genomic signatures can be used to phylogenetically classify organisms from arbitrary sampled DNA. Genomic signatures can also be used to search for horizontally transferred DNA or DNA regions subjected to special selection forces. Thus, the stability of the genomic signature can be used as a measure of genomic homogeneity. The factors associated with the stability of the genomic signatures are not known, and this motivated us to investigate further. We analyzed the intra-genomic variance of genomic signatures based on AT content normalization (0(th) order Markov model) as well as genomic signatures normalized by smaller DNA words (1(st) and 2(nd) order Markov models) for 636 sequenced prokaryotic genomes. Regression models were fitted, with intra-genomic signature variance as the response variable, to a set of factors representing genomic properties such as genomic AT content, genome size, habitat, phylum, oxygen requirement, optimal growth temperature and oligonucleotide usage variance (OUV, a measure of oligonucleotide usage bias), measured as the variance between genomic tetranucleotide frequencies and Markov chain approximated tetranucleotide frequencies, as predictors.

PRINCIPAL FINDINGS

Regression analysis revealed that OUV was the most important factor (p<0.001) determining intra-genomic homogeneity as measured using genomic signatures. This means that the less random the oligonucleotide usage is in the sense of higher OUV, the more homogeneous the genome is in terms of the genomic signature. The other factors influencing variance in the genomic signature (p<0.001) were genomic AT content, phylum and oxygen requirement.

CONCLUSIONS

Genomic homogeneity in prokaryotes is intimately linked to genomic GC content, oligonucleotide usage bias (OUV) and aerobiosis, while oligonucleotide usage bias (OUV) is associated with genomic GC content, aerobiosis and habitat.