Genomic signatures: tracing the origin of retroelements at the nucleotide level.

We investigate the nucleotide sequences of 23 retroelements (4 mammalian retroviruses, 1 human, 3 yeast, 2 plant, and 13 invertebrate retrotransposons) in terms of their oligonucleotide composition in order to address the problem of relationship between retrotransposons and retroviruses, and the coadaptation of these retroelements to their host genomes. We have identified by computer analysis over-represented 3-through 6-mers in each sequence. Our results indicate retrotransposons are heterogeneous in contrast to retroviruses, suggesting different modes of evolution by slippage-like mechanisms. Moreover, we have calculated the Observed/Expected number ratio for each of the 256 tetramers and analysed the data using a multivariate approach. The tetramer composition of retroelement sequences appears to be influenced by host genomic factors like methylase activity.