Figure 3.

Patterns of expression of L1 RNA, ORF1p, and ORF2p during spermatogenesis. Available data suggest that the synthesis of full-size L1 RNP consisting of FL-L1 RNA, ORF1p, and ORF2p is repressed during spermatogenesis. No L1-related products have been found in spermatogonia. Transient expression of FL-L1 RNA and ORF1p (but not ORF2p) occurs in spermatocyte I at the onset of meiosis (leptotene through mid-pachytene stage of prophase I). These upregulated FL-L1s are implicated in chromosome pairing [145]. Transcription of L1s resumes in spermatocyte II and continues in spermatids. However, L1 RNAs are mostly short, spliced, and prematurely polyadenylated species that translate into either ORF1p or ORF2p. Their functional significance is not known. Transcription of L1s, ORF1p, and ORF2p is downregulated by the spermatozoa stage. A small amount of FL-L1 RNA (not detectable by Northern blotting) is available in spermatozoa. ORF2p, not found in testicular spermatozoa [18], is detectable in the sub-acrosomal space in mature sperm [27].