Comparison of base composition and codon usage in insect mitochondrial genomes

Insects, the most biodiverse taxonomic group, have high AT content in their mitochondrial genomes. Although codon usage tends to be AT-rich, base composition and codon usage of mitochondrial genomes may vary among taxa. Thus, we compare base composition and codon usage patterns of 49 insect mitochondrial genomes. For protein coding genes, AT content is as high as 80% in the Hymenoptera and Lepidoptera and as low as 72% in the Orthopotera. The AT content is high at positions 1 and 3, but A content is low at position 2. A close correlation occurs between codon usage and tRNA abundance in nuclear genomes. Optimal codons can pair well with the antr codons of the most abundant tRNAs. One tRNA gene translates a synonymous codon family in vertebrate mitochondrial genomes and these tRNA anticodons can pair with optimal codons. However, optimal codons cannot pair with anticodons in mtDNA ofCochiiomyia hominivorax (Dipteral: CaLliphoridae). Ten optimal codons cannot pair with tRNA anticodons in all 49 insect mitochondrial genomes; non-optimal codon-anticodon usage is common and codon usage is not influenced by tRNA abundance.     

Comparative analysis of codon usage patterns in chloroplast genomes of the Asteraceae family

Codon usage bias (CUB) is an important evolutionary feature in a genome and has been widely documented from prokaryotes to eukaryotes. However, the significance of CUB in the Asteraceae family has not been well understood, with no Asteraceae species having been analyzed for this characteristic. Here, we use bioinformatics approaches to comparatively analyze the general patterns and influencing factors of CUB in five Asteraceae chloroplast (cp) genomes. The results indicated that the five genomes had similar codon usage patterns, showing a strong bias towards a high representation of NNA and NNT codons. Neutrality analysis showed that these cp genomes had a narrow GC distribution and no significant correlation was observed between GC₁₂ and GC₃. Parity Rule 2 (PR2) plot analysis revealed that purines were used more frequently than pyrimidines. Effective number of codons (ENc)-plot analysis showed that most genes followed the parabolic line of trajectory, but several genes with low ENc values lying below the expected curve were also observed. Furthermore, correspondence analysis of relative synonymous codon usage (RSCU) yielded a first axis that explained only a partial amount of variation of codon usage. These findings suggested that both natural selection and mutational bias contributed to codon bias, while selection was the major force to shape the codon usage in these Asteraceae cp genomes. Our study, which is the first to investigate codon usage patterns in Asteraceae plastomes, will provide helpful information about codon distribution and variation in these species, and also shed light on the genetic and evolutionary mechanisms of codon biology within this family.     

Comparative multivariate analysis of codon and amino acid usage in three Leishmania genomes

Multivariate analysis of codon and amino acid usage was performed for three Leishmania species, including L. donovani, L. infantum and L. major. It was revealed that all three species are under mutational bias and translational selection. Lower GC 12 and higher GC 3S in all three parasites suggests that the ancestral highly expressed genes (HEGs), compared to lowly expressed genes (LEGs), might have been rich in AT-content. This also suggests that there must have been a faster rate of evolution under GC-bias in LEGs. It was observed from the estimation of synonymous/non-synonymous substitutions in HEGs that the HEG dataset of L. donovani is much closer to L. major evolutionarily. This is also supported by the higher d N value as compared to d S between L. donovani and L. major, suggesting the conservation of synonymous codon positions between these two species and the role of translational selection in shaping the composition of protein-coding genes.     

Origins of highly mosaic mycobacteriophage genomes

Bacteriophages are the most abundant organisms in the biosphere and play major roles in the ecological balance of microbial life. The genomic sequences of ten newly isolated mycobacteriophages suggest that the bacteriophage population as a whole is amazingly diverse and may represent the largest unexplored reservoir of sequence information in the biosphere. Genomic comparison of these mycobacteriophages contributes to our understanding of the mechanisms of viral evolution and provides compelling evidence for the role of illegitimate recombination in horizontal genetic exchange. The promiscuity of these recombination events results in the inclusion of many unexpected genes including those implicated in mycobacterial latency, the cellular and immune responses to mycobacterial infections, and autoimmune diseases such as human lupus. While the role of phages as vehicles of toxin genes is well established, these observations suggest a much broader involvement of phages in bacterial virulence and the host response to bacterial infections.     

Relationship between base composition in non-coding DNA of genes and codon composition

The C + G percentage in third position of codons is linearly dependent on the C + G composition of flanking regions and introns. A similar relationship is shown for the first and second position which significantly influence the nature of amino acid sequence. If mutations would be oriented according to the local base composition, this will imply that genes of the same multigenic family would evolve at different rate.     

Selection on codon usage and base composition in Drosophila americana

We have used a polymorphism dataset on introns and coding sequences of X-linked loci in Drosophila americana to estimate the strength of selection on codon usage and/or biased gene conversion (BGC), taking into account a recent population expansion detected by a maximum-likelihood method. Drosophila americana was previously thought to have a stable demographic history, so that this evidence for a recent population expansion means that previous estimates of selection need revision. There was evidence for natural selection or BGC favouring GC over AT variants in introns, which is stronger for GC-rich than GC-poor introns. By comparing introns and coding sequences, we found evidence for selection on codon usage bias, which is much stronger than the forces acting on GC versus AT basepairs in introns.     

Comparative analysis of the base biases at the gene terminal portions in seven eukaryote genomes

Adenine nucleotides have been found to appear preferentially in the regions after the initiation codons or before the termination codons of bacterial genes. Our previous experiments showed that AAA and AAT, the two most frequent second codons in Escherichia coli, significantly enhance translation efficiency. To determine whether such a characteristic feature of base frequencies exists in eukaryote genes, we performed a comparative analysis of the base biases at the gene terminal portions using the proteomes of seven eukaryotes. Here we show that the base appearance at the codon third positions of gene terminal regions is highly biased in eukaryote genomes, although the codon third positions are almost free from amino acid preference. The bias changes depending on its position in a gene, and is characteristic of each species. We also found that bias is most outstanding at the second codon, the codon after the initiation codon. NCN is preferred in every genome; in particular, GCG is strongly favored in human and plant genes. The presence of the bias implies that the base sequences at the second codon affect translation efficiency in eukaryotes as well as bacteria.     

Comparative codon and amino acid composition analysis of Tritryps-conspicuous features of Leishmania major

Comparative analyses of codon/amino acid usage in Leishmania major, Trypanosoma brucei and Trypanosoma cruzi reveal that gene expressivity and GC-bias play key roles in shaping the gene composition of all three parasites, and protein composition of L. major only. In T. brucei and T. cruzi, the major contributors to the variation in protein composition are hydropathy and/or aromaticity. Principle of Cost Minimization is followed by T. brucei, disregarded by T. cruzi and opposed by L. major. Slowly evolving highly expressed gene-products of L. major bear signatures of relatively AT-rich ancestor, while faster evolution under GC-bias has characterized the lowly expressed genes of the species by higher GC12-content.     

Quantitative relationship between synonymous codon usage bias and GC composition across unicellular genomes

Background  

Codon usage bias has been widely reported to correlate with GC composition. However, the quantitative relationship between codon usage bias and GC composition across species has not been reported.     

Within-intron correlation with base composition of adjacent exons in different genomes

Within-intron difference of correlation with base composition of the adjacent exons was studied in the genomes of 34 species. For this purpose, GC-percent was determined for segments of 50 bp in length taken at both intron margins and in the internal part of the intron. It was found that in certain genomes the coefficient of correlation with GC-percent of the adjacent exon was significantly higher for the intron margin than for the internal part of the intron (homeotherms, cereals). Only part of this difference can be explained by unequal probability of insertion of transposable elements. Those multicellular organisms which have a low or no within-intron difference in correlation with the adjacent exons (anamniotes, invertebrates, dicots) show a higher local compositional heterogeneity (a greater exon/intron contrast in the GC-content). These results are evidence against the mutational bias being a possible explanation for the compositional genome heterogeneity. Thus, in the genomes with a high global heterogeneity there seems to be a selective force for compliance of intron base composition with the adjacent exons. This force is stronger in those parts of the intron that are closer to exons. In addition, the previously found positive general correlation between the genome size and average intron length was confirmed with a much larger dataset. However, within separate phylogenetic groups this rule can be broken, as it occurs in the cereals (family Poaceae), where a negative correlation was found.     

Comparative analysis of processed pseudogenes in the mouse and human genomes

Pseudogenes are important resources in evolutionary and comparative genomics because they provide molecular records of the ancient genes that existed in the genome millions of years ago. We have systematically identified approximately 5000 processed pseudogenes in the mouse genome, and estimated that approximately 60% are lineage specific, created after the mouse and human diverged. In both mouse and human genomes, similar types of genes give rise to many processed pseudogenes. These tend to be housekeeping genes, which are highly expressed in the germ line. Ribosomal-protein genes, in particular, form the largest sub-group. The processed pseudogenes in the mouse occur with a distinctly different chromosomal distribution than LINEs or SINEs - preferentially in GC-poor regions. Finally, the age distribution of mouse-processed pseudogenes closely resembles that of LINEs, in contrast to human, where the age distribution closely follows Alus (SINEs).     

Comparative analysis of variable regions in the genomes of variola virus

Nucleotide sequences of two extended segments of the terminal variable regions in variola virus genome were determined. The size of the left segment was 13.5 kbp and of the right, 10.5 kbp. Totally, over 540 kbp were sequenced for 22 variola virus strains. The conducted phylogenetic analysis and the data published earlier allowed us to find the interrelations between 70 variola virus isolates, the character of their clustering, and the degree of intergroup and intragroup variations of the clusters of variola virus strains. The most polymorphic loci of the genome segments studied were determined. It was demonstrated that that these loci are localized to either noncoding genome regions or to the regions of destroyed open reading frames, characteristic of the ancestor virus. These loci are promising for development of the strategy for genotyping variola virus strains. Analysis of recombination using various methods demonstrated that, with the only exception, no statistically significant recombinational events in the genomes of variola virus strains studied were detectable.     

Comparative analysis of GS and BK virus genomes.

Analysis of heteroduplexing between the genomes of GS virus, a BK-group virus, and the prototype BK virus revealed one region of nonhomology. Further analysis by cleavage of viral DNA with the restriction endonucleases EcoRI, HindIII, and HaeIII revealed that base changes in the GS virus genome spanned 0.6 to 0.7 map unit from the EcoRI site. Large T and small t antigens of GS virus appear to be similar in size to the BK virus antigens.     

Nearly neutrality and the evolution of codon usage bias in eukaryotic genomes

Here I show that the mean codon usage bias of a genome, and of the lowly expressed genes in a genome, is largely similar across eukaryotes ranging from unicellular protists to vertebrates. Conversely, this bias in housekeeping genes and in highly expressed genes has a remarkable inverse relationship with species generation time that varies by more than four orders of magnitude. The relevance of these results to the nearly neutral theory of molecular evolution is discussed.