CHICKEN REPEAT-1 ELEMENTS CONTAIN A POL-LIKE OPEN READING FRAME AND BELONG TO THE NON-LONG TERMINAL REPEAT CLASS OF RETROTRANSPOSONS

Chicken genomes contain almost-equal-to 30,000 chicken repeat 1 (CR1) elements scattered among single-copy sequences, but no information has yet been presented to account for how these elements could have dispersed. The fact that CR1 elements have common (although atypical) 3' ends and variable 5' truncations suggested to us that they might belong to the class of non-long terminal repeat retrotransposons that encode reverse transcriptases. From an analysis of unusually large CR1 elements, we now provide evidence for the presence of such a reverse transcriptase open reading frame. CR1 elements are distantly related to previously described non-long terminal repeat retrotransposons; however, we find that frog and torpedo ray genomes contain dispersed open reading frame segments that have >50% identity to the CR1 open reading frame. This result suggests that CR1-like elements exist in several vertebrate classes that have evolved independently for almost-equal-to 400 million years.