I Elements in Drosophila melanogaster

At the beginning of the 1970s Picard and L’Heritier reported that crosses between particular strains of Drosophila melanogaster produce progeny exhibiting genetic abnormalities. In 1976 Picard reported that the factor responsible for IR hybrid dysgenesis is a transposable element, the first discovered in Drosophila, and named it the I factor. The determination of its sequence showed that it belongs to the class of non-long terminal repeat retrotransposons (NLRs) also known as long interspersed nucleotidic elements (LINEs). The I factor is one of the models used to study the mechanism of transposition of NLRs because it can be mobilized at high frequency by dysgenic crosses, giving the opportunity to study the molecular mechanism of transposition in vivo. The D. melanogaster species can be divided into two classes of strains according to the IR system of hybrid dysgenesis, inducer (or I) and reactive (or R). I strains contain several complete and functional I factors, R strains do not. The mechanism of transposition of the I factor is thought to be related to target-primed reverse transcription (TPRT) requiring a full-length RNA intermediate. The study of deletion derivatives indicates that the sequences comprised between nucleotides 41 and 100 might be involved in the inhibition of somatic expression. The study of deletion derivatives showed that more than one region of the protein is involved in DNA binding and that the cysteine-rich motifs are not essential for this, but are required for the formation of the high molecular weight structures.