Homologs of Drosophila P transposons were mobile in zebrafish but have been domesticated in a common ancestor of chicken and human

A substantial fraction of vertebrate and invertebrate genomes is composed of mobile elements and their derivatives. One of the most intensively studied transposon families, the P elements of Drosophila, was thought to exist exclusively in the genomes of dipteran insects. Based on the data provided by the human genome project, in 2001 our group has identified a P element-homologous sequence in the human genome. This P element-homologous human gene, named Phsa, is 19,533 nucleotides long, comprises six exons and five introns, and encodes a protein of still unknown function with a length of 903 amino acid residues. The N-terminal THAP domain of the putative Phsa protein shows similarities to the site-specific DNA-binding domain of the Drosophila P element transposase. In the present study, FISH analysis and the screening of a human lambda genomic library revealed a single copy of Phsa located on the long arm of chromosome 4, upstream of a gene coding for the hypothetical protein DKFZp686L1814. The same gene arrangement was found for the homologous gene Pgga in the genome of chicken, thus, displaying Pgga at orthologous position on the long arm of chromosome 4. The single-copy gene status and the absence of terminal inverted repeats and target-site duplications indicate that Phsa and Pgga constitute domesticated stationary sequences. In contrast, a considerable number of P-homologous sequences with terminal inverted repeats and intact target-site duplications could be identified in zebrafish, strongly indicating that Pdre elements were mobile within the zebrafish genome. Pdre elements are the first P-like transposons identified in a vertebrate species. With respect to Phsa, gene expression studies showed that Phsa is expressed in a broad range of human tissues, suggesting that the putative Phsa protein plays a not yet understood but essential role in a specific metabolic pathway. We demonstrate that P-homologous DNA sequences occur in the genomes of 21 analyzed vertebrates but only as rudiments in the rodents. Finally, the evolutionary history of P element-homologous vertebrate sequences is discussed in the context of the "molecular domestication" hypothesis versus the "source gene hypothesis."     

Merlin, a new superfamily of DNA transposons identified in diverse animal genomes and related to bacterial IS1016 insertion sequences

Several new families of DNA transposons were identified by computer-assisted searches in a wide range of animal species that includes nematodes, flat worms, mosquitoes, sea squirt, zebrafish, and humans. Many of these elements have coding capacity for transposases, which are related to each other and to those encoded by the IS1016 group of bacterial insertion sequences. Although these transposases display a motif similar to the DDE motif found in many transposases and integrases, they cannot be directly allied to any of the previously described eukaryotic transposases. Other common features of the new eukaryotic and bacterial transposons include similarities in their terminal inverted repeats and 8-bp or 9-bp target-site duplications. Together, these data indicate that these elements belong to a new superfamily of DNA transposons, called Merlin/IS1016, which is common in many eubacterial and animal genomes. We also present evidence that these transposons have been recently active in several animal species. This evidence is particularly strong in the parasitic blood fluke Schistosoma mansoni, in which Merlin is also the first described DNA transposon family.     

Genomic island genes in a coastal marine Synechococcus strain confer enhanced tolerance to copper and oxidative stress

Highly variable regions called genomic islands are found in the genomes of marine picocyanobacteria, and have been predicted to be involved in niche adaptation and the ecological success of these microbes. These picocyanobacteria are typically highly sensitive to copper stress and thus, increased copper tolerance could confer a selective advantage under some conditions seen in the marine environment. Through targeted gene inactivation of genomic island genes that were known to be upregulated in response to copper stress in Synechococcus sp. strain CC9311, we found two genes (sync_1495 and sync_1217) conferred tolerance to both methyl viologen and copper stress in culture. The prevalence of one gene, sync_1495, was then investigated in natural samples, and had a predictable temporal variability in abundance at a coastal monitoring site with higher abundance in winter months. Together, this shows that genomic island genes can confer an adaptive advantage to specific stresses in marine Synechococcus, and may help structure their population diversity.     

Exploration and grading of possible genes from 183 bacterial strains by a common protocol to identification of new genes: Gene Trek in Prokaryote Space (GTPS).

A large number of complete microorganism genomes has been sequenced and submitted to the public database and then incorporated into our complete genome database, Genome Information Broker (GIB, http://gib.genes.nig.ac.jp/). However, when comparative genomics is carried out, researchers must be aware that there are protein-coding genes not confirmed by homology or motif search and that reliable protein-coding genes are missing. Therefore, we developed a protocol (Gene Trek in Prokaryote Space, GTPS) for finding possible protein-coding genes in bacterial genomes. GTPS assigns a degree of reliability to predicted protein-coding genes. We first systematically applied the protocol to the complete genomes of all 123 bacterial species and strains that were publicly available as of July 2003, and then to those of 183 species and strains available as of September 2004. We found a number of incorrect genes and several new ones in the genome data in question. We also found a way to estimate the total number of orthologous genes in the bacterial world.