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Monophyletic Origin of Alu Elements in Primates
Authors:Ewa Zi?tkiewicz  Chantal Richer  Daniel Sinnett  Damian Labuda
Institution:Centre de Recherche de l'H?pital Sainte-Justine, Centre de Cancérologie Charles Bruneau, Département de Pédiatrie de l'Université de Montréal, Montréal, Québec, H3T-1C5 Canada, CA
Abstract:To get insight into the early evolution of the primate Alu elements, we characterized sequences of these repeats from the Malagasy prosimians, lemurs (Lemuridae) and sifakas (Indriidae), as well as from galagos (Lorisidae). These sequences were compared with the oldest Alu species known from the human genome: dimeric Alu J and S and free Alu monomers. Our analysis indicates that about 60 Myr ago, before the prosimian divergence, free left and right monomers formed an Alu heterodimer connected by a 19-nucleotide-long A-rich linker. The resulting elements successfully propagated in diverging primate lineages until about ∼20 Myr ago, conserving similar sequence features and essentially the same Alu RNA secondary structure. We suggest that until that time the same ``retropositional niche', molecular machinery making possible the proliferation by retroposition, constrained the evolution of Alu elements in extant primate species. These constraints became subsequently relaxed. In the Malagasy prosimians the dimeric Alu continued to amplify after acquiring a 34- to 36-nucleotide extension of their linker segment, whereas in the galago genome the ``retropositional niche' was occupied by novel short elements. Received: 1 December 1997 / Accepted: 30 January 1998
Keywords:: Short interspersed sequence elements (SINES) —  Retroposition —  Alu evolution —  Alu RNA folding —  Primate lineages            Prosimians —  Genomic repeats
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