Examination of protein sequence homologies: III. Ribosomal protein YS25 fromSaccharomyces cerevisiae and its counterparts fromSchizosaccharomyces pombe,rat liver,andEscherichia coli

Summary The sequences of the ribosomal proteins YS25, SP-S28, RL-S21, and Ec-S6, fromSaccharomyces cerevisiae, Schizosaccharomyces pombe, rat liver, andEscherichia coli, respectively, have been examined using a computer program that searches for homologous tertiary structures. Matrices of comparisons among the eukaryotic sequences show that they match each other sequentially without any internal gaps. The average values of the correlation coefficients obtained from the comparison matrices are higher for the first halves of the sequences than for the latter halves. This result suggests that the first halves of the sequences may represent a more important domain than the latter halves. The comparison matrices between the eukaryotic and bacterial sequences of ribosomal proteins, however, do not show sequentially arranged homology, though there are six well-matching segments arranged in different orders in the two types of sequences. This implies that the eukaryotic sequences of the ribosomal protein were reconstituted by two internal transpositions and six deletions of 4–12 residues each from the ancestral sequence during the divergence between bacterial and eukaryotic genes. These findings may give insight into structural and quantitative studies of evolutionary divergence between eukaryotes and prokaryotes.     

Molecular Evolution of ldpA, a Gene Mediating the Circadian Input Signal in Cyanobacteria

The ldpA gene is an element of the cyanobacterial circadian system and mediates input to the clock. Using complete prokaryotic genomes from various public databases, I analyzed the structure and phylogeny of the ldpA genes. This gene belongs to the large superfamily of ferredoxins and has a HycB domain as a core element of its structure. In addition to this domain, ldpA has two conserved terminal domains that are specific to this gene and have no homologs in the databases. All three domains are under different selective constraints. The ldpA tree topology features two very distinct clades that are essentially the same as those in the previously reported trees of the sasA gene and the kaiBC operon, two other elements of the circadian system. The data on the ldpA polymorphism and evolutionary patterns give further support to the existence of two types of the system, kaiABC- and kaiBC-based, respectively. Each type has specific functional and selective constraints, which have likely been attained through highly concordant evolution of the systems components.Reviewing Editor: Dr. Rasmus Nielsen     

Phylogeny of Prokaryotes and Chloroplasts Revealed by a Simple Composition Approach on All Protein Sequences from Complete Genomes Without Sequence Alignment

The complete genomes of living organisms have provided much information on their phylogenetic relationships. Similarly, the complete genomes of chloroplasts have helped to resolve the evolution of this organelle in photosynthetic eukaryotes. In this paper we propose an alternative method of phylogenetic analysis using compositional statistics for all protein sequences from complete genomes. This new method is conceptually simpler than and computationally as fast as the one proposed by Qi et al. (2004b) and Chu et al. (2004). The same data sets used in Qi et al. (2004b) and Chu et al. (2004) are analyzed using the new method. Our distance-based phylogenic tree of the 109 prokaryotes and eukaryotes agrees with the biologists tree of life based on 16S rRNA comparison in a predominant majority of basic branching and most lower taxa. Our phylogenetic analysis also shows that the chloroplast genomes are separated to two major clades corresponding to chlorophytes s.l. and rhodophytes s.l. The interrelationships among the chloroplasts are largely in agreement with the current understanding on chloroplast evolution.Reviewing Editor: Dr. John Oakeshott     

Genomic GC level, optimal growth temperature, and genome size in prokaryotes

Two years ago, we showed that positive correlations between optimal growth temperature (T(opt)) and genome GC are observed in 15 out of the 20 families of prokaryotes we analyzed, thus indicating that "T(opt) is one of the factors that influence genomic GC in prokaryotes". Our results were disputed, but these criticisms were demonstrated to be mistaken and based on misconceptions. In a recent report, Wang et al. [H.C. Wang, E. Susko, A.J. Roger, On the correlation between genomic G+C content and optimal growth temperature in prokaryotes: data quality and confounding factors, Biochem. Biophys. Res. Commun. 342 (2006) 681-684] criticize our results by stating that "all previous simple correlation analyses of GC versus temperature have ignored the fact that genomic GC content is influenced by multiple factors including both intrinsic mutational bias and extrinsic environmental factors". This statement, besides being erroneous, is surprising because it applies in fact not to ours but to the authors' article. Here, we rebut the points raised by Wang et al. and review some issues that have been a matter of debate, regarding the influence of environmental factors upon GC content in prokaryotes. Furthermore, we demonstrate that the relationship that exists between genome size and GC level is valid for aerobic, facultative, and microaerophilic species, but not for anaerobic prokaryotes.     

Comparative Analysis of the glg Operons of Pectobacterium chrysanthemi PY35 and Other Prokaryotes

A chromosomal region of Pectobacterium chrysanthemi PY35 that contains of genes for glycogen synthesis was isolated from a cosmid library. The operon consists of glycogen branching enzyme (glgB), glycogen debranching enzyme (glgX), ADP-glucose pyrophosphorylase (glgC), glycogen synthase (glgA), and glycogen phosphorylase (glgP) genes. Gene organization is similar to that of Escherichia coli. The purified ADP-glucose pyrophosphorylase (GlgC) was activated by fructose 1,6-bisphosphate and inhibited by AMP. The constructed glgX::Omega mutant failed to integrate into the chromosome of P. chrysanthemi by marker exchange. Phylogenetic analysis based on the 16S rDNA and the amino acid sequence of Glg enzymes showed correlation with other bacteria. gamma-Proteobacteria have the glgX gene instead of the bacilli glgD gene in the glg operon. The possible evolutionary implications of the results among the prokaryotes are discussed.     

Stable low molecular weight RNA analyzed by staircase electrophoresis, a molecular signature for both prokaryotic and eukaryotic microorganisms

Low-molecular weight RNA (LMW RNA) analysis using staircase electrophoresis was performed for several species of eukaryotic and prokaryotic microorganisms. According to our results, the LMW RNA profiles of archaea and bacteria contain three zones: 5S RNA, class 1 tRNA and class 2 tRNA. In fungi an additional band is included in the LMW RNA profiles, which correspond to the 5.8S RNA. In archaea and bacteria we found that the 5S rRNA zone is characteristic for each genus and the tRNA profile is characteristic for each species. In eukaryotes the combined 5.8S and 5S rRNA zones are characteristic for each genus and, as in prokaryotes, tRNA profiles are characteristic for each species. Therefore, stable low molecular weight RNA, separated by staircase electrophoresis, can be considered a molecular signature for both prokaryotic and eukaryotic microorganisms. Analysis of the data obtained and construction of the corresponding dendrograms afforded relationships between genera and species; these were essentially the same as those obtained with 16S rRNA sequencing (in prokaryotes) and 18S rRNA sequencing (in eukaryotes).     

低等白蚁肠道共生微生物的多样性及其功能

低等白蚁肠道里存在着复杂的微生物区系,包括真核微生物鞭毛虫和原核生物,细菌及古细菌。低等白蚁的后肠以特别膨大的囊形胃及其氢氧浓度的明显梯度分布和丰富的微生物区系为特征,是白蚁进行木质纤维素消化的主要器官。后肠内的鞭毛虫能将纤维素水解并发酵为乙酸,二氧化碳和氢,为白蚁提供营养和能源。系统发育研究表明,低等白蚁肠道共生细菌的主要类群为白蚁菌群1、螺旋体、拟杆菌,低G C mol%含量的革兰氏阳性菌和紫细菌等。而古细菌主要为甲烷短杆菌属的产甲烷菌。共生原核生物与二氧化碳的还原和氮的循环等代谢有关。但肠道共生微生物的具体功能和作用机制还有待进一步的揭示。     

Examination of protein sequence homologies: II. SixEscherichia coli L7/L12-type ribosomal “A” protein sequnces from eukaryotes and metabacteria,contrasted with those from prokaryotes

Summary Four complete and three partial sequences ofE. coli L7/L12-type ribosomal A proteins obtained from four eukaryotes (Saccharomyces cerevisiae, Artemia salina, rat liver, and wheat germ), two metabacteria (Halobacterium cutirubrum andMethanobacterium thermoautotrophicum), and the prokaryoteEscherichia coli have been compared using a computer program that searches for homologous tertiary structures. Comparison matrices show that eukaryotic sequences sequentially match each other if deletions and/or insertions of certain residues (gaps) are assumed at specific sites corresponding to residues 36, 51, 72, and 94 ofS. cerevisiae protein YL44c. This is similar to what was previously found in prokaryotes. Metabacteria, which exhibit eukaryote-type sequences, must have separated from the eukaryotes in ancient times, because an additional deletion site is found in their sequences and their sequences have low correlation coefficients with those of all the other eukaryotes. When the eukaryote-type A proteins (110–111 residues) are compared withE. coli L7/L12 (120 residues) four groups of well-matching segments are found. It was deduced that the eukaryote-type A proteins had regenerated from the prokaryote types by a transposition and several deletions, resulting in the eukaryote-type lengths. The correspondence between the eukaryotic and prokaryotic proteins, as well as that among eukaryotic proteins themselves, is discussed in terms of protein evolution.In addition, ribosomal protein YL35 fromS. cerevisiae has been compared with RL37 from rat liver, with results indicating five well-matching parts separated by four gaps, one of which consists of 20 residues. These results contrasts with those previously reported by Lin et al. No prokaryotic counterparts to these ribosomal proteins have yet been identified.     

Genomic analyses and the origin of the eukaryotes

The availability of whole-genome data has created the extraordinary opportunity to reconstruct in fine details the 'tree of life'. The application of such comprehensive effort promises to unravel the enigmatic evolutionary relationships between prokaryotes and eukaryotes. Traditionally, biologists have represented the evolutionary relationships of all organisms by a bifurcating phylogenetic tree. But recent analyses of completely sequenced genomes using conditioned reconstruction (CR), a newly developed gene-content algorithm, suggest that a cycle graph or 'ring' rather than a 'tree' is a better representation of the evolutionary relationships between prokaryotes and eukaryotes. CR is the first phylogenetic-reconstruction method to provide precise evidence about the origin of the eukaryotes. This review summarizes how the CR analyses of complete genomes provide evidence for a fusion origin of the eukaryotes.     

Biased Usages of Arginines and Lysines in Proteins Are Correlated with Local-Scale Fluctuations of the G + C Content of DNA Sequences

Amino acid residues arginine (R) and lysine (K) have similar physicochemical characteristics and are often mutually substituted during evolution without affecting protein function. Statistical examinations on human proteins show that more R than K residues are used in the proximity of R residues, whereas more K than R are used near K residues. This biased use occurs on both a global and a local scale (shorter than ∼100 residues). Even within a given exon, G + C-rich and A + T-rich short DNA segments preferentially encode R and K, respectively. The biased use of R and K on a local scale is also seen in Saccharomyces cerevisiae and Caenorhabdidtis elegans, which lack global-scale mosaic structures with varying GC%, or isochores. Besides R and K, several amino acids are also used with a positive or negative correlation with the local GC% of third codon bases. The local-, or ``within-gene'-, scale heterogeneity of the DNA sequence may influence the sequence of the encoded protein segment. Received: 2 March 1998 / Accepted: 23 April 1998