INSERTIONS OF A NOVEL CLASS OF TRANSPOSABLE ELEMENTS WITH A STRONG TARGET SITE PREFERENCE AT THE R-LOCUS OF MAIZE

The r locus of maize regulates anthocyanin synthesis in various tissue s of maize through the production of helix-loop-helix DNA binding prot eins capable of inducing expression of structural genes in the anthocy anin biosynthetic pathway. The complex r variant, R-r:standard (R-r), undergoes frequent mutation through a variety of mechanisms including displaced synapsis and crossing over, and intrachromosomal recombinati on. Here we report a new mechanism for mutation at the R-r complex: in sertion of a novel family of transposable elements. Because the elemen ts were first identified in the R-p gene of the R-r complex, they have been named P Instability Factor (PIF). Two different PIF elements wer e cloned and found to have identical sequences at their termini but di vergent internal sequences. In addition, the PIF elements showed a mar ked specificity of insertion sites. Six out of seven PIF-containing de rivatives examined had an element inserted at an identical location. T wo different members of the PIF element family were identified at this position. The seventh PIF-containing derivative examined had the elem ent inserted at a distinct position within r. Even at this location, h owever, the element inserted into a conserved target sequence. The tim ing of PIF excision is unusual. Germinal excision rates can range up t o several percent of progeny. Yet somatic sectors are rare, even in li nes exhibiting high germinal reversion rates.