Retroposition - A Persistent and Pervasive Force in Genome Evolution

Jürgen Brosius
Institute of Experimental Pathology/Molecular Neurobiology
University of Münster
Von-Esmarch-Str. 56
D-48149 Münster, Germany
telephone: 0251 835 8511
fax: 0251 835 8512
prestype: Platform
presenter: Jürgen Brosius

During transition from the RNP world (when RNA carried genetic information and RNA as well as protein had structural and catalytic assignments in the cell) to modern cells, the conversion of genetic information into DNA was probably achieved by retroposition i.e. reverse transcription of RNA into DNA and integration into the nascent DNA genome(1). One may assume that after all genomic RNA had been transformed into DNA, this process should have ceded. On the contrary, in many lineages this process is still rampant: all types of RNAs(2) can be reverse transcribed and their cDNA copies reintegrated into genomes as retronuons (a nuon is any stretch of a definable nucleic acid sequence(3). About 40% of the human genome consist of discernible retronuons. If one extrapolates this figure to "decayed", non- discernible retronuons, it is conceivable that a very large proportion of the 95-97% non- coding segments (including intergenic regions and introns) is of retronuon origin. In most cases this process leads to "junk DNA". However, there are many examples where mRNA-derived retronuons gave rise to active genes, often with different expression patterns, when compared to their respective founder genes(3,4). Retronuons derived from small non-messenger RNAs (snmRNAs) can give rise to novel snmRNA genes (such as neuronal dendritic BC1 and BC200 RNAs) and, quite often, are exapted (recruited) as regulatory elements that may alter expression or processing of targeted genes(3,5) (for compilations see Thus, retronuons are a major driving force of evolution. For example, comparison of the human genome with that of its closest relative, the chimpanzee, will reveal that neither species will contain many additional genes ­ and if, they will not differ much from the genes they originated from via duplication. Instead, we will observe exaptation of novel exons (often involving alternative splicing) from previously nonaptive intronic and flanking sequences (naptonuons) originally generated by retroposition and different patterns of expression of otherwise shared genes with respect to development and cell-type specificity, often being influenced by de novo insertions of retronuons.

  1. Brosius, J. (1999) Transmutation of tRNA over time. Nat. Genet. 22, 8-9.
  2. Brosius, J. (1999) RNAs from all categories generate retrosequences that may be exapted as novel genes or regulatory elements. Gene 238, 115-134.
  3. Brosius, J., Gould, S.J. (1992) On Genomenclature: A comprehensive (and respectful) taxonomy for pseudogenes and other 'junk DNA'. Proc. Natl. Acad. Sci. U.S.A. 89, 10706- 10710.
  4. Brosius, J. (1991) Retroposons - seeds of evolution. Science 251, 753.
  5. Brosius, J. (1999) Genomes were forged by massive bombardments with retroelements and retrosequences. Genetica 107, 209-238.

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