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内容記述 |
Cis-regulatory element is a functional unit controlling gene expression spatially and temporarily. Acquirement, retention and/or loss of cis-regulatory elements during evolution can alter gene expression patterns. In particular, cis-elements of transcription factors expressed in developmental process, as typified by tool-kit genes, would be a key to understand evolutionary novelty that emerged through vertebrate evolution.<br /> ParaHox genes are tool-kit genes that are controlling anterior-posterior body axis determination in vertebrates. ParaHox genes play important roles for digestive gut differentiation in both vertebrates and invertebrates. The ParaHox gene clusters consist of three classes of homeobox containing genes, Gsh1/2, Pdx1, and Cdx1/2/4. There are four ParaHox clusters in a mammalian genomes, most likely as a consequence of two round whole genome duplications. Despite their importance in development, cis-regulatory mechanisms of ParaHox gene clusters, particularly Gsh genes that function in segmented anterior brain in vertebrates, have not been studied well. My analysis and previous studies suggested that evolutionarily conserved sequences in ParaHox non-coding regions are strong candidates as tissue specific cis-regulatory elements. In this study, to search for cis-regulatory elements in ParaHox gene clusters, genomic sequence comparison was conducted by using diverse species of vertebrates. Furthermore cis-regulatory candidates were functionally tested by transgenic mice experiments to clarify their enhancer activities. <br /> I compared genomic sequences of the vertebrate Gsh1-Pdx1-Cdx2 cluster. Multiple <u>c</u>on served <u>n</u>on-coding <u>s</u>equences are identified in <u>P</u>araHox clusters by this analysis, and they were termed as CNSPs. The 1012bp CNSP#1, the most evolutionarily conserved element within the Gsh1-Pdx1-Cdx2 cluster, showed the highly reproducible reporter gene expression pattern in central nervous system that was similar to endogenous mRNA expression pattern of Gsh1 gene. <br /> Another highly conserved sequence among vertebrates was found in the paralogous Gsh2 gene cluster, too. One of the conserved sequences termed CNSG2#2/#4was found in similar relative position to the Gsh gene coding region. The 1246bpCNSG2#2/#4 drove lacZ expression largely similar to Gsh2 endogenous expression in anterior brain.<br /> Interestingly the expression pattern of CNSG2#2/#4 was more similar to that ofCNSP#1 than its neighboring Gsh2 gene. This prompted me to search for any sequence similarity between CNSP#1 and CNSG2#2/#4, and then I found an approximately 200bpconserved core sequence between these elements. Based on the synteny and the phylogenetic relationship between these clusters, I conclude that CNSP#1 andCNSG2#2/#4 were generated by whole genome duplication event during the ancestral vertebrate era and the core homologous 200bp sequence have been preserved for some functional reason. Paralogous conservation between CNSP#1 and CNSG2#2/#4 strongly suggests that the core conserved 200bp element already existed in the ancestral primitive vertebrate. If we assume that the core sequence is directly related to the cis-regulatory function in segmented anterior brain, we could suppose that the ancestral primitive vertebrate already had a segmented brain. Function of the shared 200bp element betweenCNSP#1 and CNSG2#2/#4 should be tested in the future, and that will not only clarify its biological function in extant vertebrates but also will give a hint to understand the timing of emergence of evolutionary novelty in ancestral vertebrates. |
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