Evidence for a multistep control in transposition of I factor in Drosophila melanogaster.

Drosophila melanogaster strains belong to one of two interactive categories, inducer (I) or reactive (R), with respect to the I-R system of hybrid dysgenesis. The dysgenic interaction results from the presence of several transposition-competent copies of a LINE-like element, the I factor, only in the genome of I strains. When a cross is performed between I males and R females, I factor transposes at high frequency in the germ line of F1 daughters, known as SF females. This transposition burst results in the sterility of SF females. I factor transposes by reverse transcription of a full-length transcript. Specific RT-PCR experiments were done to compare the amount of I factor transcript in samples corresponding to various transposition frequencies. The sensitivity of the method allowed the ready detection of the I factor RNA in every tissue and genetic background examined. Comparison of amplification signals suggests that I factor activity in ovaries is regulated at different levels. First, the amount of I factor RNA subjected to negative and positive regulation. Whereas the negative control, which limits transposition in nonpermissive contexts, may be exerted by an I factor encoded repressor function, the positive control is linked to reactivity level, a cellular state maternally inherited from R mothers. Additionally, negative regulation is also exerted downstream of I factor RNA. This differs notably from previous conclusions in which transcription was envisaged as the main level of regulation of the I factor transposition.