P-element

P elements are transposable elements that were discovered in Drosophila as the causative agents of genetic traits called hybrid dysgenesis. The transposon is responsible for P trait of P element and it is found only in wild flies.

All P elements have a canonical structure contain 31 bp terminal inverted repeats and 11 bp internal inverted repeats located at THAP domain of the transposase. The shorter and longest P elements are nonautonomous elements. The longest P elements encode transposase needed for transposition.

In hybrid dysgenesis, one strain of Drosophila mates with another to produce hybrid offspring cause chromosomal damage known to be dysgenic. Hybrid dysgenesis requires a contribution from both parents. For example, in the P-M system, P strain contributing paternal and M strain contributing maternal. The reverse cross, with M father and P mother, produces normal offspring, as it crosses P x P or M x M manner. P male chromosome can cause dysgenesis when cross with an M female.

P element also encodes a suppressor of transposition, which accumulates in the cytoplasm during the development of cells. Thus, in a cross of a P or M male with a P female, the female cytoplasm contains the suppressor, which binds to any P elements and prevents their transposition.

P elements are commonly used as mutagenic agents in genetic experiments with Drosophila. One advantage of this approach is that the mutations are easy to locate .

The P element encodes for the protein P transposase and is flanked by terminal inverted repeats which are important for its mobility. Unlike laboratory strain females, wild type females are thought to express an inhibitor to P transposase function. This inhibitor reduces the disruption to the genome caused by the P elements, allowing fertile progeny. Evidence for this comes from crosses of laboratory females (which lack P transposase inhibitor) with wild type males (which have P elements). In the absence of the inhibitor, the P elements can proliferate throughout the genome, disrupting many genes and killing progeny.

The P element is a class II transposon, and moves by a DNA-based "cut and paste" mechanism. The sequence comprises 4 exons with 3 introns. Complete splicing of the introns produces the transposase enzyme, while alternative partial splicing of intron 1 and 2 leaving in only intron 3 encodes the P element repressor. The complete, autonomous P element encodes a transposase enzyme, which recognizes the 31 bp terminal inverted repeats of the P element and catalyzes P element excision and re-insertion. The complete element is 2907 bp; non-autonomous P elements contain an internal deletion of varying length which abolishes transposase production, but such elements can still be mobilized if transposase is encoded elsewhere in the genome. P element insertion and subsequent excision results in the production of 8 bp direct repeats, and the presence of such repeats is indicative of previous P element activity.

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