UC Berkeley

Efficient P element transposition is dependent on the expression of a transposon-encoded transposase and the recruitment of endogenous Drosophila DNA repair proteins to sites of transposase induced DNA breaks. Previous biochemical analysis of proteins bound to the 31 bp inverted repeats identified an 18 kDa basic leucine zipper protein (bZIP) (CG6272) termed Inverted Repeat Binding Protein 18 (IRBP18). IRBP18 forms a functional heterodimer with another bZIP protein Xrp1/CG17836, which is sufficient to bind the 31bp terminal inverted repeats in vitro. Together these proteins, in cell culture models, repress transcription from the P element promoter, as well as facilitate efficient DNA repair after transposase-mediated cleavage. Additionally flies homozygous for null mutations of IRBP18 show increased killing after somatic mobilization of P elements: consistent with an inability to repair post cleavage DNA breaks. In the absence of P elements, IRBP18-null flies are male sterile and display increased sensitivity to DNA damaging agents. IRBP18/Xrp1 heterodimer forms the site-specific binding core of a larger IRBP complex; of which several of the co-purifying proteins function in RNA interference (RNAi) pathways. Recently RNAi pathways have been shown to be integral components of general genome maintenance due to regulation of heterochromatin formation of repeated DNA elements, such as ribosomal DNA genes. In addition in this thesis Rm62, the novel role of the DEAD/H-box RNA helicase, in DNA repair is examined. Flies homozygous for a hypomorphic allele of Rm62 display similar sensitivity to DNA damaging agents as IRBP18 mutants. Interestingly, the majority of the DNA repair phenotype of Rm62 is caused by a nucleolar localized isoform of the protein. Furthermore RNAi-knockdown of the Rm62 interaction partner, Argonaute 2, also resulted in decrease in DNA repair efficiency. Taken together, these data implicate the IRBP complex as a critical component in maintaining genomic integrity and thereby connecting a new site-specific DNA binding protein complex to genome stability and DNA repair.