Transcripts homologous to a long repeated DNA element in the human genome

Using three cloned DNA fragments from a 6.4-kb (kilobase pair) DNA element repeated several thousand times in the human genome (Adams, J. W., Kaufman, R. E., Kretschmer, P. J., Harrison, M. and Nienhuis, A. W. (1980) Nucleic Acids Res. 8, 6113-6128) and DNA/RNA hybridization, we show that transcripts homologous to this DNA family exist in total cellular RNA from human blood cells and from a mouse-human hybrid cell line with one human chromosome, the X. No such transcripts were detected in RNA from rabbit blood or a mouse cell line. For each DNA fragment studied, we found that blood transcripts and X-chromosome transcripts were indistinguishable in electrophoretic mobility and very heterogeneous in length; in addition, prominent hybridization bands were seen at 4.7 and 1.9 kb. Transcription from this DNA family likely occurs from heterogeneous templates. The existence of RNAs smaller than 6.4 kb suggests that part of the repeat unit can be transcribed and/or there exists a cellular mechanism to make these short RNAs from longer precursors. The vast majority of the RNAs homologous to the long repeat are not polyadenylated. In blood RNA there are a few hundred copies of beta-globin mRNA for every transcript homologous to this 6.4 kb repeat.