Molecular organization and evolution of the D17Leh80-like loci in the mouse t complex.

We determined the sequence of Tu80, one of the molecular clones derived from microdissected fragments of Chromosome (Chr) 17. The sequence data demonstrated that Tu80 contains an open reading frame (ORF) of 204 bp. Two sequences within the ORF, one homologous to the LINE1 element and the other to the first intron of the C epsilon gene of mouse immunoglobulin, were observed. A separate sequence, homologous to Tu80, designated as NOV1, was isolated from a genomic library of mouse Chr 17. NOV1 was found to contain an inserted B2 repeat, making it structurally different from Tu80. The sequences of Tu80 and NOV1 were compared with those of LINE1 and the first intron of the C epsilon gene. The results suggest that the ancestor of the Tu80-like sequence might have arisen through recombination between a LINE1 element and the C epsilon gene. It is concluded that Tu80 and NOV1 might have resulted from duplication of an ancestral sequence followed by divergence. The comparative analysis also demonstrated a high degree of conservation of the LINE1-like sequence in Tu80 and NOV1. Based on the structure of human, rat, rabbit, and mouse LINE1 fragments, as well as those of NOV1 and Tu80, a phylogenetic tree was constructed. The available data tend to support the assumption that the ancestor for the Tu80-like sequence might have arisen not later than 27-33 million years ago.