P element
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A P element is a transposon that is present in the fruit fly Drosophila melanogaster and is used widely for mutagenesis and the creation of genetically modified flies used for genetic research. The P element causes a phenotype known as hybrid dysgenesis. They seem to have first appeared in the species only in the middle of the twentieth century. Within 50 years, they have spread through every wild population of the species, so that only older laboratory stocks lack them.
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[edit] Characteristics
The P element is a class II transposon, which means that its movement within the genome is made possible by a transposase. The complete element is 2907 bp and is autonomous because it encodes a functional transposase; non-autonomous P elements which lack a functional transposase gene due to mutation also exist. Non-autonomous P elements can still move within the genome if there are autonomous elements to produce transposase. The P element can be identified by its terminal 31-bp inverted repeats, and the 8 bp direct repeats in movement into and out of DNA sequence produces.
[edit] Hybrid dysgenesis
Hybrid dysgenesis refers to the high rate of mutation in germ line cells of Drosophila strains resulting from a cross of males with autonomous P elements (P Strain/P cytotype) and females that lack P elements (M Strain/M cytotype). The hybrid dysgenesis syndrome is marked by temperature-dependent sterility, elevated rates of mutation, chromosome rearrangement and recombination. This is caused by the transposition of P elements within the germ-line cells of offspring of P strain males with M strain females. This transposition only occurs in germ-line cells because a splicing event needed to make transposase mRNA does not occur in somatic cells.
The reason that hybrid dysgenesis takes place when crossing P strain males with M strain females and not when crossing P strain females (females with autonomous P elements) and M strain males is that the eggs of P strain females contain high amounts of repressor protein that prevents transcription of the transposase gene. The eggs of M strain mothers, on the other hand, do not contain the repressor protein, allowing for transposition of P elements from the sperm of fathers.
[edit] P element in molecular biology
The P element has found wide use in Drosophila research as a mutagen. The mutagenesis system typically uses an autonomous but immobile element, and a mobile nonautonomous element. Flies from subsequent generations can then be screened by phenotype or PCR.
The most common use of the P element is for genetic modification of Drosophila. The gene of interest is created as a recombinant DNA sequence with P element insertion sequences added at the '5 and '3. A plasmid containing the sequence is then injected into a pre-blastoderm embryo in the presence of the transposase and it will transpose into the genome.
[edit] References
- Leland Hartwell et al.. 2004. Genetics - From Genes to Genomes 2nd Edition. McGraw-Hill
- Engels, W. R. P Elements in Drosophila
