Rate matrices for analyzing large families of protein sequences.

We propose and study a new approach for the analysis of families of protein sequences. This method is related to the LogDet distances used in phylogenetic reconstructions; it can be viewed as an attempt to embed these distances into a multidimensional framework. The proposed method starts by associating a Markov matrix to each pairwise alignment deduced from a given multiple alignment. The central objects under consideration here are matrix-valued logarithms L of these Markov matrices, which exist under conditions that are compatible with fairly large divergence between the sequences. These logarithms allow us to compare data from a family of aligned proteins with simple models (in particular, continuous reversible Markov models) and to test the adequacy of such models. If one neglects fluctuations arising from the finite length of sequences, any continuous reversible Markov model with a single rate matrix Q over an arbitrary tree predicts that all the observed matrices L are multiples of Q. Our method exploits this fact, without relying on any tree estimation. We test this prediction on a family of proteins encoded by the mitochondrial genome of 26 multicellular animals, which include vertebrates, arthropods, echinoderms, molluscs, and nematodes. A principal component analysis of the observed matrices L shows that a single rate model can be used as a rough approximation to the data, but that systematic deviations from any such model are unmistakable and related to the evolutionary history of the species under consideration.

HIV-1 and HIV-2 LTR nucleotide sequences: assessment of the alignment by N-block presentation, "retroviral signatures" of overrepeated oligonucleotides, and a probable important role of scrambled stepwise duplications/deletions in molecular evolution.

Previous analyses of retroviral nucleotide sequences, suggest a so-called "scrambled duplicative stepwise molecular evolution" (many sectors with successive duplications/deletions of short and longer motifs) that could have stemmed from one or several starter tandemly repeated short sequence(s). In the present report, we tested this hypothesis by focusing on the long terminal repeats (LTRs) (and flanking sequences) of 24 human and 3 simian immunodeficiency viruses. By using a calculation strategy applicable to short sequences, we found consensus overrepresented motifs (often containing CTG or CAG) that were congruent with the previously defined "retroviral signature." We also show many local repetition patterns that are significant when compared with simply shuffled sequences. First- and second-order Markov chain analyses demonstrate that a major portion of the overrepresented oligonucleotides can be predicted from the dinucleotide compositions of the sequences, but by no means can biological mechanisms be deduced from these results: some of the listed local repetitions remain significant against dinucleotide-conserving shuffled sequences; together with previous results, this suggests that interspersed and/or local mononucleotide and oligonucleotide repetitions could have biased the dinucleotide compositions of the sequences. We searched for suggestive evolutionary patterns by scrutinizing a reliable multiple alignment of the 27 sequences. A manually constructed alignment based on homology blocks was in good agreement with the polypeptide alignment in the coding sectors and has been exhaustively assessed by using a multiplied alphabet obtained by the promising mathematical strategy called the N-block presentation (taking into account the environment of each nucleotide in a sequence). Sector by sector, we hypothesize many successive duplication/deletion scenarios that fit our previous evolutionary hypotheses. This suggests an important duplication/deletion role for the reverse transcriptase, particularly in inducing stuttering cryptic simplicity patterns.

Codon usage as a tool to predict the cellular location of eukaryotic ribosomal proteins and aminoacyl-tRNA synthetases.

In spite of many efforts, the prediction of the location of proteins in eukaryotic cells (cytoplasm, mitochondrion or chloroplast) is still far from straightforward. In some cases (e.g. ribosomal proteins and aminoacyl-tRNA synthetases) both the cytoplasmic proteins and their organellar counterparts are encoded by the nuclear genome. A factorial correspondence analysis of the codon usage in yeast and Caenorhabditis elegans shows that the codon usage of those nuclear genes encoding ribosomal proteins or aminoacyl-tRNA synthetases is markedly different, depending on the final location of the proteins (cytoplasmic or mitochondrial). As a consequence, the location of such proteins-whose sequences are now frequently determined by systematic genomic sequencing-can be easily and quickly predicted. A WWW interface has been developed, aimed at providing a user-friendly tool for codon usage pattern analysis. It is available from http://www.genetique.uvsq.fr/afc.html

Evolution of genes, evolution of species: the case of aminoacyl-tRNA synthetases.

All of the aminoacyl-tRNA synthetase (aaRS) sequences currently available in the data banks have been subjected to a systematic analysis aimed at finding gene duplications, genetic recombinations, and horizontal transfers. Evidence is provided for the occurrence (or probable occurrence) of such phenomena within this class of enzymes. In particular, it is suggested that the monomeric PheRS from the yeast mitochondrion is a chimera of the alpha and beta chains of the standard tetrameric protein. In addition, it is proposed that the dimeric and tetrameric forms of GlyRS are the result of a double and independent acquisition of the same specificity within two different subclasses of aaRS. The phylogenetic reconstructions of the evolutionary histories of the genes encoding aaRS are shown to be extremely diverse. While large segments of the population are consistent with the broad grouping into the three Woesian domains, some phylogenetic reconstructions do not place the Archae and the Eucarya as sister groups but, rather, show a gram-negative bacteria/eukaryote clustering. In addition, many individual genes pose difficulties that preclude any simple evolutionary scheme. Thus, aaRS's are clearly a paradigm of F. Jacob's "odd jobs of evolution" but, on the whole, do not call into question the evolutionary scenario originally proposed by Woese and subsequently refined by others.

Gene 2 of the sigma rhabdovirus genome encodes the P protein, and gene 3 encodes a protein related to the reverse transcriptase of retroelements.

The nucleotide sequence of the genes 2 and 3 of the Drosophila rhabdovirus sigma was determined from cDNAs to viral genome and poly(A)+ mRNAs. Gene 2 comprises 1032 nucleotides and contains a long ORF encoding a molecular weight 35,208 polypeptide present in infected cells and in virions which migrates in SDS-PAGE as a doublet of M(r) about 60 kDa. The distribution of acidic charges as well as the electrophoretic properties of the protein are characteristic of the rhabdovirus P proteins. Gene 3 comprises 923 nucleotides and contains a long ORF capable of coding a polypeptide of 298 amino acids of MW 33,790. The putative protein (PP3) is similar in size to a minor component of the virions. Computer analysis shows that the sequence of PP3 contains three motifs related to the conserved motifs of reverse transcriptases.