RODENT L1 EVOLUTION HAS BEEN DRIVEN BY A SINGLE DOMINANT LINEAGE THATHAS REPEATEDLY ACQUIRED NEW TRANSCRIPTIONAL REGULATORY SEQUENCES

All mammalian genomes contain approximately 100,000 copies of the tran sposable element LINES-1 (L1). Phylogenetic analysis indicates that th e L1 progenitor predates the mammalian radiation; since that time, the open reading frames encoded in L1 have evolved under selection. The l east conserved regions within L1 are the 5'-terminal transcriptional r egulatory sequences. In rodents, four types of L1 elements (A, F, and V from mouse and R from rat) have been defined according to the type o f apparently nonhomologous promoter sequence present at the 5' end. In this study, we investigate the relationships between these four types of promoters. DNA sequence was determined from approximately 1.5-kb r egions from the 5' ends of seven F- and three V-type L1 elements. Thes e sequences were aligned with 29 previously reported L1 elements. Phyl ogenetic analysis was then performed on the homologous regions of the alignment. The results indicate that in mouse all of the A-, F-, and V -type elements belong to a single dominant lineage but were inserted i nto the genome during different time periods; V-type elements are the oldest, while A-type elements are the most recently inserted. V-type e lements also appear ancestral to the R-type elements found in rat and therefore were replicatively competent prior to the divergence of rat and mouse. Analysis of sequence identity indicates that the different 5' promoters did not derive from a common ancestor. Therefore, the dom inant L1 lineage appears to have acquired novel promoter sequences fro m non-L1 sources. Transposable elements from a wide range of species s how similar structural rearrangements, suggesting that acquisition of new sequences may be a common theme in their evolution.