Computational Analysis and Paleogenomics of Interspersed ...

Computational Analysis of Interspersed Repeats | 45Consensus sequences and ancestral repeat reconstructionThe consensus sequence is generated from a multiple alignment of a sampleof interspersed copies assigned to the same TE family using a simple majorityrule for each base position in the alignment (Fig. 2.3). The consensus may befurther improved by manually correcting ambiguous or overlooked positionsthat could introduce stop codons and frameshifts in the ORF(s). Given thatall TEs from a family arose from a single or few active TE copies (also knownas source or master elements), the consensus can be considered a particularlyaccurate approximation of the sequence of the original master copy that gaverise to the family under consideration ( Jurka and Milosavljevic, 1991; Smit etal., 1995). This principle has been experimentally validated through the demonstrationthat the reconstructed consensus sequence of three different animalDNA transposase families restored their enzymatic activity in vitro and/or invivo (Ivics et al., 1997; Lampe et al., 1996; Miskey et al., 2003). Another earlysuccessful exploitation of the consensus approach was the resurrection of anextinct promoter for the human L1 retrotransposon based on a phylogeneticallyreconstructed consensus (Adey et al., 1994).An extension of the concept of consensus is that sequence comparisons ofindividual repeat copies to their consensus can be used to infer an approximateage of the repeat family, i.e. the time lapsed since transposition and insertionof the copies (Kapitonov and Jurka, 1996; Smit et al., 1995). This is because atCopy 1Copy 2Copy 3Copy 4Copy 5Copy 6Copy 7Copy 8CONS.66756756Figure 2.3 The concept of TE consensus sequence and its utility to infer the age of aTE family. In this schematic representation, eight related TE sequences are shown asthick horizontal lines. The consensus (CONS.) is deduced from a multiple alignmentof TE sequences that vary at several Figure positions. 3 Most sequence variations occur atdiscrete positions, but some are also shared between copies. All shared and nonsharednucleotide changes may be used as phylogenetic characters to infer the relationshipof the elements within the family (see Figure 2.4). Changes include single nucleotidesubstitutions (vertical ticks) and insertions (triangle above the sequence) and deletions(interrupted line) of variable size. The number of pairwise nucleotide substitutionsto the consensus is shown for each copy on the right hand side of the figure. Theaverage number of substitution to the consensus (here n = 6) can be used to infer theapproximate age of the family if the neutral rate of nucleotide substitution per year isknown for the host species. For example, if the neutral rate is 1 substitution per site permillion year, the family depicted here would be ~6 million year old.Uncorrected proofs — not for distribution