Close relationship between non-viral retroposons inDrosophila melanogaster

G elements constitute one of the several moderately repeated DNA families of the Drosophila melanogaster genome. G elements lack terminal repetitions and structurally resemble mammalian processed pseudogenes because they terminate at one end in oligo-A tracts of variable length. G elements are mostly interspersed in the chromocentric heterochromatin with other repeated DNA sequences. Nucleotide sequence analysis of G3A, a family member inserted in a non-nucleolar rDNA unit, shows that functional G elements might have coding capacity for two polypeptides; one has homology to reverse transcriptases, the other is reminiscent of RNA binding proteins derived from the cleavage of retroviral gag polyproteins. Functionally related polypeptides are similarly encoded by members of two other Drosophila repeated DNA families, the F elements and the I factors. The similarity in structural organization and the relatedness of their potential gene products favors the hypothesis that G, F, and I sequences derive from a common ancestor and result from processes based on the reverse transcription of RNA intermediates that probably differ markedly from those ensuring the maintenance and dispersion of copia-like elements.