Genomic characterization of recent human LINE-1 insertions: Evidence supporting random insertion

LINE-1 (L1) elements play ail important creative role in genomic evolution by distributing both L1 and non-L1 DNA in a process called retrotranspositi on. A large percentage of the human genome consists of DNA that has been di spersed by the L1 transposition machinery. L1 elements are not randomly dis tributed in genomic DNA but are concentrated in re.-ions with lower GC cont ent. In an effort to understand the consequences of L1 insertions, we have begun an investigation of their genomic characteristics and the changes tha t occur to them over time. We compare human L1 insertions that were created either during recent human evolution or during the primate radiation. We r eport that L1 insertions are an important source for the creation of new mi crosatellites. We provide evidence that L1 first strand cDNA synthesis can occur from an internal priming event. We note that in contrast to older L1 insertions, recent Us are distributed randomly in genomic DNA, and the shif t in the L1 genomic distribution Occurs relatively rapidly. Taken together, our data indicate that strong forces act on newly inserted L1 retrotranspo sons to alter their structure and distribution.