Some extrachromosomal circular DNAs containing the Alu family of dispersed repetitive sequences may be reverse transcripts.

A 300 base-pair (bp) size class of small polydisperse circular DNA (spcDNA) isolated from the BSC-1 line of African Green monkey kidney cells was cleaved with the restriction endonuclease Sau3A, and the resulting fragments (100 to 200 bp) were cloned in bacteriophage M13 mp7. The nucleotide sequence of each of 24 clones containing DNA sequences homologous to the Alu family of mobile, dispersed, repetitive elements was then determined. Analysis of these sequences revealed that many, and perhaps all, of the 300 bp Alu-containing spcDNAs had regions in which the 5' and 3' ends of the normal Alu element were juxtaposed and covalently joined. Although more than one model can explain the generation of such circular molecules, the most attractive one at this time involves their generation from reverse transcripts of Alu-specific RNAs.