Selection for highly biased amino acid frequency in the TolA cell envelope protein of Proteobacteria

The GC Content as a Main Factor Shaping the Amino Acid Usage During Bacterial Evolution Process

Understanding how proteins evolve is important, and the order of amino acids being recruited into the genetic codons was found to be an important factor shaping the amino acid composition of proteins. The latest work about the last universal common ancestor (LUCA) makes it possible to determine the potential factors shaping amino acid compositions during evolution. Those LUCA genes/proteins from Methanococcus maripaludis S2, which is one of the possible LUCA, were investigated. The evolutionary rates of these genes positively correlate with GC contents with P-value significantly lower than 0.05 for 94% homologous genes. Linear regression results showed that compositions of amino acids coded by GC-rich codons positively contribute to the evolutionary rates, while these amino acids tend to be gained in GC-rich organisms according to our results. The first principal component correlates with the GC content very well. The ratios of amino acids of the LUCA proteins coded by GC rich codons positively correlate with the GC content of different bacteria genomes, while the ratios of amino acids coded by AT rich codons negatively correlate with the increase of GC content of genomes. Next, we found that the recruitment order does correlate with the amino acid compositions, but gain and loss in codons showed newly recruited amino acids are not significantly increased along with the evolution. Thus, we conclude that GC content is a primary factor shaping amino acid compositions. GC content shapes amino acid composition to trade off the cost of amino acids with bases, which could be caused by the energy efficiency.

Molecular evolution of NASP and conserved histone H3/H4 transport pathway

BACKGROUND

NASP is an essential protein in mammals that functions in histone transport pathways and maintenance of a soluble reservoir of histones H3/H4. NASP has been studied exclusively in Opisthokonta lineages where some functional diversity has been reported. In humans, growing evidence implicates NASP miss-regulation in the development of a variety of cancers. Although a comprehensive phylogenetic analysis is lacking, NASP-family proteins that possess four TPR motifs are thought to be widely distributed across eukaryotes.

RESULTS

We characterize the molecular evolution of NASP by systematically identifying putative NASP orthologs across diverse eukaryotic lineages ranging from excavata to those of the crown group. We detect extensive silent divergence at the nucleotide level suggesting the presence of strong purifying selection acting at the protein level. We also observe a selection bias for high frequencies of acidic residues which we hypothesize is a consequence of their critical function(s), further indicating the role of functional constraints operating on NASP evolution. Our data indicate that TPR1 and TPR4 constitute the most rapidly evolving functional units of NASP and may account for the functional diversity observed among well characterized family members. We also show that NASP paralogs in ray-finned fish have different genomic environments with clear differences in their GC content and have undergone significant changes at the protein level suggesting functional diversification.

CONCLUSION

We draw four main conclusions from this study. First, wide distribution of NASP throughout eukaryotes suggests that it was likely present in the last eukaryotic common ancestor (LECA) possibly as an important innovation in the transport of H3/H4. Second, strong purifying selection operating at the protein level has influenced the nucleotide composition of NASP genes. Further, we show that selection has acted to maintain a high frequency of functionally relevant acidic amino acids in the region that interrupts TPR2. Third, functional diversity reported among several well characterized NASP family members can be explained in terms of quickly evolving TPR1 and TPR4 motifs. Fourth, NASP fish specific paralogs have significantly diverged at the protein level with NASP2 acquiring a NNR domain.

Evolutionary conservation of amino acid composition in paralogous insect vitellogenins

Comparison of paralogous vitellogenins in 10 insect species representing six orders showed a remarkable degree of conservation of amino acid composition in spite of sequence differences. For example, the correlation between the percentages of the 20 amino acids in two vitellogenins from the beetle Tribolium castaneum was 0.975, even though the two amino acid sequences differed from each other at 49.4% of sites. There was a positive correlation between the frequency of occurrence of reciprocal pairs of amino acids in more distantly related paralogs, and this correlation was generally strongest when both of the amino acids in the pair were nutritionally essential. These results imply that conservation of amino acid composition occurs through amino acid replacements that result in a balanced loss and gain of each amino acid residue. Thus insect vitellogenins seem to be subject to an unusual kind of purifying selection, where the amino acid content is conserved rather than the sequence per se, selection apparently arising from the nutritional needs of the developing embryo appears to be responsible for maintaining the balance of amino acids.

Evolutionary dynamics of the 5S rDNA gene family in the mussel Mytilus: mixed effects of birth-and-death and concerted evolution

In higher eukaryotes, the gene family encoding the 5S ribosomal RNA (5S rRNA) has been used (together with histones) to showcase the archetypal example of a gene family subject to concerted evolution. However, recent studies have revealed conspicuous features challenging the predictions of this model, including heterogeneity of repeat units, the presence of functional 5S gene variants as well as the existence of 5S rDNA divergent pseudogenes lacking traces of homogenization. In the present work, we have broadened the scope in the evolutionary study of ribosomal gene families by studying the 5S rRNA family in mussels, a model organism which stands out among other animals due to the heterogeneity it displays regarding sequence and organization. To this end, 48 previously unknown 5S rDNA units (coding and spacer regions) were sequenced in five mussel species, leading to the characterization of two new types of units (referred to here as small-beta 5S rDNA and gamma-5S rDNA) coexisting in the genome with alpha and beta rDNA units. The intense genetic dynamics of this family is further supported by the first description of an association between gamma-5S rDNA units and tRNA genes. Molecular evolutionary and phylogenetic analyses revealed an extensive lack of homology among spacer sequences belonging to different rDNA types, suggesting the presence of independent evolutionary pathways leading to their differentiation. Overall, our results suggest that the long-term evolution of the 5S rRNA gene family in mussels is most likely mediated by a mixed mechanism involving the generation of genetic diversity through birth-and-death, followed by a process of local homogenization resulting from concerted evolution in order to maintain the genetic identities of the different 5S units, probably after their transposition to independent chromosomal locations.

Birth-and-death long-term evolution promotes histone H2B variant diversification in the male germinal cell line

The rich diversity within each of the five histone families (H1, H2A, H2B, H3, and H4) can hardly be reconciled with the notion of homogenizing evolution. The prevalence of birth-and-death long-term evolution over concerted evolution has already been demonstrated in the linker histone H1 family as well as for the H2A, H3, and H4 core histone families. However, information about histone H2B is lacking. In the present work, we have analyzed the diversity of the members of this histone family across different eukaryotic genomes and have characterized the mechanisms involved in their long-term evolution. Our results reveal that, quite in contrast with other histones, H2B variants are subject to a very rapid process of diversification that primarily affects the male germinal cell lineage and involves their functional specialization probably as a consequence of neofunctionalization and subfunctionalization events after gene duplication. The overall parallelism observed between the molecular phylogenies and the relationships among the electrostatic potentials of the different variants suggests that the latter may have played a major structural selective constraint during H2B evolution. It thus seems that the reorganization of chromatin structure during spermiogenesis might have affected the evolutionary constraints driving histone H2B evolution, leading to an increase in diversity.

Molecular phylogeny, long-term evolution, and functional divergence of flavin-containing monooxygenases

Flavin-containing monooxygenases (FMOs) metabolize xenobiotic compounds, many of which are clinically important, as well as endogenous substrates as part of a discrete physiological process. The FMO gene family is conserved and ancient with representatives present in all phyla so far examined. However, there is a lack of information regarding the long-term evolution and functional divergence of these proteins. This study represents the first attempt to characterize the long-term evolution followed by the members of this family. Our analysis shows that there is extensive silent divergence at the nucleotide level suggesting that this family has been subject to strong purifying selection at the protein level. Invertebrate FMOs have a polyphyletic origin. The functional divergence of FMOs 1-5 started before the split between amphibians and mammals. The vertebrate FMO5 is more ancestral than other four FMOs. Moreover, the existence of higher levels of codon bias was detected at the N-terminal ends, which can be ascribed to the critical role played by the FAD binding motif in this region. Finally, critical amino acid residues for FMO functional divergence (type I & II) after gene duplication were detected and characterized.

The evolutionary differentiation of two histone H2A.Z variants in chordates (H2A.Z-1 and H2A.Z-2) is mediated by a stepwise mutation process that affects three amino acid residues

Background

The histone H2A family encompasses the greatest number of core histone variants of which the replacement variant H2A.Z is currently one of the most heavily studied. No clear mechanism for the functional variability that H2A.Z imparts to chromatin has yet been proposed. While most of the past studies have referred to H2A.Z generically as a single protein, in vertebrates it is a mixture of two protein forms H2A.Z-1 (previously H2A.Z) and H2A.Z-2 (previously H2A.F/Z or H2A.V) that differ by three amino acids.

Results

We have performed an extensive study on the long-term evolution of H2A.Z across metazoans with special emphasis on the possible selective mechanisms responsible for the differentiation between H2A.Z-1 and H2A.Z-2. Our results reveal a common origin of both forms early in chordate evolution. The evolutionary process responsible for the differentiation involves refined stepwise mutation change within the codons of the three differential residues. This eventually led to differences in the intensity of the selective constraints acting upon the different H2A.Z forms in vertebrates.

Conclusion

The results presented in this work definitively reveal that the existence of H2A.Z-1 and H2A.Z-2 is not a whim of random genetic drift. Our analyses demonstrate that H2A.Z-2 is not only subject to a strong purifying selection but it is significantly more evolutionarily constrained than H2A.Z-1. Whether or not the evolutionary drift between H2A.Z-1 and H2A.Z-2 has resulted in a functional diversification of these proteins awaits further research. Nevertheless, the present work suggests that in the process of their differently constrained evolutionary pathways, these two forms may have acquired new or complementary functions.

Distribution and evolution of short tandem repeats in closely related bacterial genomes

Simultaneous identification and comparison of perfect and imperfect microsatellites within a genome is a valuable tool both to overcome the lack of a consensus definition of SSRs and to assess repeat history. Detailed analysis of the overall distribution of perfect and imperfect microsatellites in closely related bacterial taxa is expected to give new insight into the evolution of prokaryotic genomes. We have performed a genome-wide analysis of microsatellite distribution in four Escherichia coli and seven Chlamydial strains. Chlamydial strains generally have a higher density of SSRs and show greater intra-group differences of SSR distribution patterns than E. coli genomes. In most investigated genomes the distribution of the total lengths of matching perfect and imperfect trinucleotide repeats are highly similar, with the notable exception of C. muridarum. Closely related strains show more similar repeat distribution patterns than strains separated by a longer divergence time. The discrepancy between the preferred classes of perfect and imperfect repeats in C. muridarum implies accelerated evolution of SSRs in this particular strain. Our results suggest that microsatellites, although considerably less abundant than in eukaryotic genomes, may nevertheless play an important role in the evolution of prokaryotic genomes and several gene families.

New insights into the nucleophosmin/nucleoplasmin family of nuclear chaperones

Basic proteins and nucleic acids are assembled into complexes in a reaction that must be facilitated by nuclear chaperones in order to prevent protein aggregation and formation of non-specific nucleoprotein complexes. The nucleophosmin/nucleoplasmin (NPM) family of chaperones [NPM1 (nucleophosmin), NPM2 (nucleoplasmin) and NPM3] have diverse functions in the cell and are ubiquitously represented throughout the animal kingdom. The importance of this family in cellular processes such as chromatin remodeling, genome stability, ribosome biogenesis, DNA duplication and transcriptional regulation has led to the rapid growth of information available on their structure and function. The present review covers different aspects related to the structure, evolution and function of the NPM family. Emphasis is placed on the long-term evolutionary mechanisms leading to the functional diversification of the family members, their role as chaperones (particularly as it pertains to their ability to aid in the reprogramming of chromatin), and the importance of NPM2 as an essential component of the amphibian chromatin remodeling machinery during fertilization and early embryonic development.

Long-term evolution and functional diversification in the members of the nucleophosmin/nucleoplasmin family of nuclear chaperones

The proper assembly of basic proteins with nucleic acids is a reaction that must be facilitated to prevent protein aggregation and formation of nonspecific nucleoprotein complexes. The proteins that mediate this orderly protein assembly are generally termed molecular (or nuclear) chaperones. The nucleophosmin/nucleoplasmin (NPM) family of molecular chaperones encompasses members ubiquitously expressed in many somatic tissues (NPM1 and -3) or specific to oocytes and eggs (NPM2). The study of this family of molecular chaperones has experienced a renewed interest in the past few years. However, there is a lack of information regarding the molecular evolution of these proteins. This work represents the first attempt to characterize the long-term evolution followed by the members of this family. Our analysis shows that there is extensive silent divergence at the nucleotide level suggesting that this family has been subject to strong purifying selection at the protein level. In contrast to NPM1 and NPM-like proteins in invertebrates, NPM2 and NPM3 have a polyphyletic origin. Furthermore, the presence of selection for high frequencies of acidic residues as well as the existence of higher levels of codon bias was detected at the C-terminal ends, which can be ascribed to the critical role played by these residues in constituting the acidic tracts and to the preferred codon usage for phosphorylatable amino acids at these regions.