Similar Target Site Selection Occurs in Integration of Plant and Mammalian Retroposons
The reverse transcription of RNA in DNA is responsible for the generation of large families of repetitive sequences called retroposons or non-LTR retrotransposons. Recent reports established that the integration of mammalian SINE and LINE retroposons occurs at nonrandom staggered breaks, probably resulting from the action of a LINE-encoded endonuclease (Feng et al. 1996; Jurka 1997; Jurka et al. 1998). We report here that plant SINE S1 retroposons also integrate at nonrandom staggered breaks. One of the two nicks involved in S1 integration is associated mainly with the 5′-Y/AAANNNG-3′ motif. The other nick at opposite DNA strand occurs preferably within 14–16 bp, a situation also observed for mammalian retroposons, but is not associated with any specific motif. Further studies on the distribution of dinucleotides surrounding the two nicking sites showed that, as for mammalian retroposons, S1 retroposons integrate at sites rich in TA, CA, and TG dinucleotides. These dinucleotides were reported as specific DNA sites where special DNA structures called ``kinks'' may occur under bending constraints. Nicking sites are preceded by peaks in frequency of di-pyrimidine followed by peaks of di-purine. These results suggest that the general A/T richness of a given DNA region and the presence of short runs of pyrimidines followed by short runs of purines could represent a favorable context for the integration of retroposons. In such a context, an endonuclease upon fixation could be able to generate the kink at the pyrimidine/purine transition and to nick the DNA. The similarities in target site selection observed for plant and mammalian retroposons suggest that retroposition is a surprisingly well conserved process.
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