Developmental and cell type specificity of LINE-1 expression in mouse testis: implications for transposition.

The LINE-1, or L1, family of interspersed repeated DNA constitutes roughly 10% of the mammalian genome. Its abundance is due to duplicative transposition via an RNA intermediate, L1-encoded proteins, and reverse transcription. Although, in principle, transposition may occur in any cell type, expression and transposition of a full-length functional element in the germ line are necessary to explain the evolutionary genetics of L1. We have found differential expression of L1 protein and RNA in germ and somatic cells of the mouse testis during development. Of particular interest is the coexpression of full-length, sense-strand L1 RNA and L1-encoded protein in leptotene and zygotene spermatocytes at postnatal day 14 of development. Expression in meiotic prophase precedes the strand breakage that occurs during chromosomal recombination; this offers an avenue for L1 insertion into new locations in chromosomal DNA in a cell type that ensures L1 propagation in future generations.