RT Journal
A1 Boeke, Jef D.
T1 The Unusual Phylogenetic Distribution of Retrotransposons: A Hypothesis
JF Genome Research 
JO Genome Research 
YR 2003 
FD September 01 
VO 13 
IS 9 
SP 1975 
OP 1983 
DO 10.1101/gr.1392003 
UL http://genome.cshlp.org/content/13/9/1975.abstract 
AB Retrotransposons have proliferated extensively in eukaryotic lineages; the  genomes of many animals and plants comprise 50% or more retrotransposon  sequences by weight. There are several persuasive arguments that the enzymatic  lynchpin of retrotransposon replication, reverse transcriptase (RT), is an  ancient enzyme. Moreover, the direct progenitors of retrotransposons are  thought to be mobile self-splicing introns that actively propagate themselves  via reverse transcription, the group II introns, also known as retrointrons.  Retrointrons are represented in modern genomes in very modest numbers, and  thus far, only in certain eubacterial and organellar genomes. Archaeal genomes  are nearly devoid of RT in any form. In this study, I propose a model to  explain this unusual distribution, and rationalize it with the proposed  ancient origin of the RT gene. A cap and tail hypothesis is proposed. By this  hypothesis, the specialized terminal structures of eukaryotic mRNA provide the  ideal molecular environment for the lengthening, evolution, and subsequent  massive expansion of highly mobile retrotransposons, leading directly to the  retrotransposon-cluttered structure that typifies modern metazoan genomes and  the eventual emergence of retroviruses.