Environmental factors affecting transcription of the human L1 retrotransposon. I. Steroid hormone-like agents

The L1 retrotransposon has significantly shaped the structure of the human genome. At least 30% of human genome sequence can be attributed to L1 reverse transcriptase activity. There are 105 copies of the human L1 retrotransposon, L1Hs, most of which are defective, although ~8–9x103 are full length. L1Hs elements transpose through an RNA intermediate and transcription is thought to be the rate limiting step in retrotransposition. Because transcription of retrotransposons in a variety of organisms has been shown to respond to environmental stimuli, we investigated the influence of various agents on transcription from two different L1Hs promoters. The activity of the L1Hs promoters was analyzed by transfecting L1Hs-expressing cell lines with plasmids containing the L1Hs promoters fused to the LacZ reporter gene and monitoring expression with a ß-galactosidase assay. Small increases in ß-galactosidase activity were observed with both L1Hs promoters after treatment with serum, testosterone, dihydrotestosterone and organochloride pesticides, indicating that these agents can influence L1Hs transcription.

Environmental factors affecting transcription of the human L1 retrotransposon. II. Stressors

Retrotransposons have clearly molded the structure of the human genome. The reverse transcriptase coded for by long interspersed nuclear elements (LINEs) accounts for 35% of the human genome, with 8–9 x 105 copies of the most common human LINE element, L1Hs. Retrotransposons cycle through an RNA intermediate with transcription as the rate limiting step. Because various retrotransposons have been demonstrated to be induced by environmental stimuli, we investigated the response of the L1Hs promoter to various agents. L1Hs promoter activity was analyzed by transfecting an L1Hs-expressing cell line with plasmids containing one of two L1Hs promoters fused to the LacZ reporter gene. L1Hs promoter activity was then monitored with a ß-galactosidase assay. Treatment with UV light and heat shock resulted in a small increase in ß-galactosidase activity from one promoter, while treatment with tetradecanoylphorbol 13-acetate resulted in small increases in ß-galactosidase activity from both promoters. No increase in ß-galactosidase activity was observed after exposure to X-rays or hydrogen peroxide.