Solitary human endogenous retroviruses-K LTRs retain transcriptional activity in vivo, the mode of which is different in different cell types

Solitary long terminal repeats (LTRs) of human endogenous retroviruses (HER Vs), tens of thousands of which are spread all over the genome, contain a v ariety of potential transcription regulatory elements. Information on trans criptional behavior of individual solitary LTRs, however, is limited. We st udied the transcriptional activity of several individual HERV-K LTRs in a v ariety of tissues and cell lines. The RT-PCR technique targeted at specific amplification of the U3 or U5 regions of individual LTRs together with the ir unique genomic flanks was used to estimate the content of each region in the transcripts. An unequal abundance of the U3 and U5 regions in the tran scripts of the same LTR in different cells and tumors was observed. Each LT R is transcribed differently in different cells or tissues, and transcripti onal behavior of different LTRs was different in the same cell line or tiss ue. The transcriptional status of LTRs varies in response to mitogenic and stress factors and in tumor tissues compared to normal counterparts. The LT Rs thus seem to be the subjects of specific transcription regulation. The d ata obtained indicate that an appreciable fraction of the LTRs retained reg ulatory potential throughout millions of years of evolution and thus may co ntribute to the overall transcription regulatory network. (C) 2001 Academic Press.