Related polypeptides are encoded by Drosophila F elements, I factors, and mammalian L1 sequences.

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Abstract
The structural organization of Drosophila F elements closely resembles that of L1 sequences, a major family of repetitive DNA elements dispersed in the genome of all mammals. Members of both families are flanked by target-site duplications of different length, vary in size because of heterogeneity at one end, and invariably terminate at the other end in characteristic adenosine-rich stretches often preceded by polyadenylylation signals. The nucleotide sequence of Fw, an F element found in the white locus of wi+A flies, reveals a large open reading frame upstream of the 3' adenosine-rich terminus encoding a possible reverse transcriptase homologous to those potentially encoded by functional L1 units and Drosophila I factors. A cysteine-rich region within an interrupted frame located at the 5' terminus of Fw suggests that complete F elements might additionally encode a nucleic acid binding protein. The observation that F elements and I factors encode functionally related polypeptides, and the extensive similarity of their hypothetical reverse transcriptases to L1 open reading frames, favors the hypothesis that all these sequences are evolutionarily related and transpose upon the cDNA conversion of RNA intermediates.