Reconstructing Genome Trees of Prokaryotes Using Overlapping Genes

• Main Authors: Cheng, Chih-Hsien, 鄭智先
• Other Authors: Lu, Chin Lung
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/63384854247052601688

碩士 === 國立交通大學 === 生物資訊研究所 === 97 === Overlapping genes (OGs) are defined as adjacent genes whose coding sequences overlap partially or entirely. In fact, they are ubiquitous in microbial genomes and more conserved between species than non-overlapping genes. Based on this property, we have previously implemented a web server, named OGtree, that allows the user to reconstruct genome trees of some prokaryotes according to their pairwise OG distances. By analogy to the analyses of gene content and gene order, the OG distance between two genomes we defined was based on a measure of combining OG content (i.e., the normalized number of shared orthologous OG pairs) and OG order (i.e. the normalized OG breakpoint distance) in their whole genomes. A shortcoming of using the concept of breakpoints to define the OG distance is its inability to analyze the OG distance of multi-chromosomal genomes. In addition, the amount of orthologous overlapping coding sequences between some distantly related prokaryotic genomes may be limited so that it is hard to find enough orthologous OGs to properly evaluate their pairwise OG distances. In this study, we therefore define a new OG order distance that is based on more biologically accurate rearrangements (e.g., reversals, transpositions and translocations) rather than breakpoints and that is applicable to both uni-chromosomal and multi-chromosomal genomes. In addition, we expand the term ”gene” to include both its coding sequence and regulatory regions so that two adjacent genes whose coding sequences or regulatory regions overlap with each other are considered as a pair of overlapping genes. This is because overlapping of regulatory regions of distinct genes suggests that the regulation of expression for these genes should be more or less interrelated. Based on these modifications, we have reimplemented our OGtree as a new web server OGtree2.0 and have also evaluated its accuracy of genome tree reconstruction on a testing dataset consisting of 21 Proteobacteria genomes. Our experimental results have finally shown that our current OGtree2.0 indeed outperforms its previous version OGtree, as well as another similar server BPhyOG, significantly in the quality of genome tree reconstruction, because the phylogenetic tree obtained by OGtree2.0 is greatly congruent with the reference tree that coincides with the taxonomy accepted by biologists for these Proteobacteria.