A RETROTRANSPOSON OF THE NON-LONG TERMINAL REPEAT CLASS FROM THE HUMAN BLOOD FLUKE SCHISTOSOMA-MANSONI - SIMILARITIES TO THE CHICKEN-REPEAT-1-LIKE ELEMENTS OF VERTEBRATES

The genomes of representative species of fishes, amphibians, and repti les contain non-long-terminal-repeat (non-LTR) retrotransposons showin g strong sequence identity to the chicken repeat 1 (CR1) non-LTR retro transposon from birds. These nonavian retroelements have been termed C R1-like elements. We have isolated sequences of a non-LTR retrotranspo son from the human blood fluke Schistosoma mansoni. These schistosome sequences, which we have termed the SR1 family of non-LTR retrotranspo sons, contain regions of deduced amino acids characteristic of the CR1 -like elements. SR1 elements possess atypical 3' termini consisting of the tandem repeat (AACCATTTG)(2) which are similar in structure to th e imperfect tandem repeat of the 3' termini of CR1. There are at least 200 copies of SR1 interspersed through the genome of S. mansoni. The structural and amino acid sequence similarities of SR1 with members of the CR1-like elements suggest that the SR1 family belongs to the CR1- like category of non-LTR retrotransposons. Although other non-LTR retr otransposons have been described in invertebrates, this is the first C R1-like element reported from a nonvertebrate taxon, suggesting that t he phylogenetic distribution of CR1-like retrotransposons is not restr icted to vertebrates.