Regulation of P element transposition in Drosophila melanogaster.

摘要

P elements are transposable elements found in Drosophila melanogaster . Full-length 2.9 kb P elements encode an 87 kD transposase protein and a truncated, 66 kD repressor protein. The transposase protein catalyzes the cut-and-paste transposition of P elements, which is regulated by the maternally-inherited P cytotype state.;P cytotype regulation involves a competition between transposase and repressor proteins for binding to their common sites at the P element ends. Another aspect of P cytotype regulation is transcripitional repression of the P element promoter. Initially, it was believed that binding of the 66 kD repressor protein to its site at the 5' P element end, which overlaps the P element promoter, resulted in P cytotype transcriptional repression. However, we showed in Chapter 2 that non-P element promoters that did not contain binding sites for the 66 kD repressor protein, and that were contained within P element ends, were also transcriptionally repressed by P cytotype. The data suggested that P cytotype transcriptional repression may occur by a chromatin-based transcriptional silencing mechanism that is independent of the 66 kD repressor protein.;This conclusion is supported by the experiments described in Chapter 3. We showed that maternally-inherited natural or recombinant P elements inserted in subtelomeric heterochromatin transcriptionally silenced a IacZ reporter transgene. Since the recombinant P elements could not encode the 66 kD repressor protein, our data proved that P cytotype transcriptional repression is independent of this repressor protein. Furthermore, insertion of the recombinant P elements within subtelomeric heterochromatin was required for transcriptional silencing of the IacZ reporter transgene. These data indicated that the chromatin enviroment surrounding P elements is an important aspect of P cytotype transcriptional repression.;P element transposition may also be regulated by a Drosophila protein, the inverted repeat binding protein (IRBP), which binds to the essential, 31 bp inverted repeats present at the P elimnent termini. The latest candidate for IRBP is an 18 kD protein that is homologous to CCAAT/enhancer binding proteins, as described in Chapter 4. However, it has not been proven that the 18 kD protein is IRBP as recombinant 18 kD protein did not exhibit IRBP activity.